If you want to know which beamer colors are used in certain theme, look at its definition style file, beamerthemeMarsburg.sty in this case. There you will see:
\mode<presentation>
\usecolortheme{whale}
\setbeamercolor*{titlelike}{use=structure,fg=structure.fg}
\useoutertheme[height=0pt]{sidebar}
\setbeamercolor{sidebar}{parent=palette primary}
{\usebeamercolor{palette quaternary}}
{\usebeamercolor{palette primary}}
\setbeamertemplate{sidebar canvas \beamer@sidebarside}[vertical shading][top=palette quaternary.bg,bottom=palette primary.bg]
which means that this theme uses color theme whale and that sidebar has a vertical shading based in colors palette quaternary.bg and palette primary.bg.
Now you have two possibilities, change palette definitions or just change sidebar definition. You must use the first one in case you want to be sure that every template based on this color is also change. If you just want to change sidebar background it's enough to add
\definecolor{myBlue}{RGB}{45,170,250}
\makeatletter
\setbeamertemplate{sidebar canvas right}[vertical shading][top=palette quaternary.bg,bottom=myBlue]
\makeatother
in your preamble. Of course, if your side bar is on the left, change right by left.
The result looks like:
The complete code is:
\documentclass{beamer}
% beamer theme
\usetheme{Marburg}
% german spelling
\usepackage[english,ngerman]{babel}
% set font encoding
\usepackage[T1]{fontenc}
% math stuff
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
% color
%\usepackage{xcolor}
% set font to Helvetica Light
\usepackage{xltxtra}
\setsansfont[Ligatures=TeX]{Tahoma}
\title{Präsentation}
\author{Max Mustermann}
\date{\today}
% removes navigation bar
\beamertemplatenavigationsymbolsempty
\definecolor{myBlue}{RGB}{45,170,250}
\makeatletter
\setbeamertemplate{sidebar canvas right}[vertical shading][top=palette quaternary.bg,bottom=myBlue]
\makeatother
\begin{document}
\begin{frame}
\frametitle{Ein Demotitel}
abcdefghijklmnopqrstuvwxyz äöüß 1234567890
ABCDEFGHIJKLMNOPQRSTUVWXYZ
\end{frame}
\end{document}
Notes:
- I've used Tahoma font instead of original Helvetica Light.
beamer already load xcolor, you can delete \usepackage{xcolor} from your preamble.
Result of \listfiles command
*File List*
beamer.cls 2013/12/02 3.33 A class for typesetting presentations (rcs-revi
sion 332bfd3ce558)
beamerbasercs.sty 2013/12/25 (rcs-revision 31cc758a62ae)
beamerbasemodes.sty 2013/09/03 (rcs-revision 768f2d98ca64)
beamerbasedecode.sty 2010/05/01 (rcs-revision efa082c6111d)
ifpdf.sty 2011/01/30 v2.3 Provides the ifpdf switch (HO)
beamerbaseoptions.sty 2013/03/10 (rcs-revision 47431932db0d)
keyval.sty 2014/05/08 v1.15 key=value parser (DPC)
geometry.sty 2010/09/12 v5.6 Page Geometry
ifvtex.sty 2010/03/01 v1.5 Detect VTeX and its facilities (HO)
ifxetex.sty 2010/09/12 v0.6 Provides ifxetex conditional
geometry.cfg
size11.clo 2007/10/19 v1.4h Standard LaTeX file (size option)
pgfcore.sty 2010/04/11 v3.0.0 (rcs-revision 1.7)
graphicx.sty 2014/04/25 v1.0g Enhanced LaTeX Graphics (DPC,SPQR)
graphics.sty 2009/02/05 v1.0o Standard LaTeX Graphics (DPC,SPQR)
trig.sty 1999/03/16 v1.09 sin cos tan (DPC)
graphics.cfg 2007/01/18 v1.5 graphics configuration of teTeX/TeXLive
xetex.def 2014/04/28 v4.01 LaTeX color/graphics driver for XeTeX (RRM/JK)
pgfsys.sty 2013/11/30 v3.0.0 (rcs-revision 1.47)
pgfrcs.sty 2013/12/20 v3.0.0 (rcs-revision 1.28)
everyshi.sty 2001/05/15 v3.00 EveryShipout Package (MS)
pgfrcs.code.tex
pgfsys.code.tex
pgfsyssoftpath.code.tex 2013/09/09 (rcs-revision 1.9)
pgfsysprotocol.code.tex 2006/10/16 (rcs-revision 1.4)
xcolor.sty 2007/01/21 v2.11 LaTeX color extensions (UK)
color.cfg 2007/01/18 v1.5 color configuration of teTeX/TeXLive
pgfcore.code.tex
xxcolor.sty 2003/10/24 ver 0.1
atbegshi.sty 2011/10/05 v1.16 At begin shipout hook (HO)
infwarerr.sty 2010/04/08 v1.3 Providing info/warning/error messages (HO)
ltxcmds.sty 2011/11/09 v1.22 LaTeX kernel commands for general use (HO)
hyperref.sty 2012/11/06 v6.83m Hypertext links for LaTeX
hobsub-hyperref.sty 2012/04/25 v1.12 Bundle oberdiek, subset hyperref (HO)
hobsub-generic.sty 2012/04/25 v1.12 Bundle oberdiek, subset generic (HO)
hobsub.sty 2012/04/25 v1.12 Construct package bundles (HO)
ifluatex.sty 2010/03/01 v1.3 Provides the ifluatex switch (HO)
intcalc.sty 2007/09/27 v1.1 Expandable calculations with integers (HO)
etexcmds.sty 2011/02/16 v1.5 Avoid name clashes with e-TeX commands (HO)
kvsetkeys.sty 2012/04/25 v1.16 Key value parser (HO)
kvdefinekeys.sty 2011/04/07 v1.3 Define keys (HO)
pdftexcmds.sty 2011/11/29 v0.20 Utility functions of pdfTeX for LuaTeX (HO)
pdfescape.sty 2011/11/25 v1.13 Implements pdfTeX's escape features (HO)
bigintcalc.sty 2012/04/08 v1.3 Expandable calculations on big integers (HO)
bitset.sty 2011/01/30 v1.1 Handle bit-vector datatype (HO)
uniquecounter.sty 2011/01/30 v1.2 Provide unlimited unique counter (HO)
letltxmacro.sty 2010/09/02 v1.4 Let assignment for LaTeX macros (HO)
hopatch.sty 2011/06/24 v1.1 Wrapper for package hooks (HO)
xcolor-patch.sty 2011/01/30 xcolor patch
atveryend.sty 2011/06/30 v1.8 Hooks at the very end of document (HO)
refcount.sty 2011/10/16 v3.4 Data extraction from label references (HO)
hycolor.sty 2011/01/30 v1.7 Color options for hyperref/bookmark (HO)
auxhook.sty 2011/03/04 v1.3 Hooks for auxiliary files (HO)
kvoptions.sty 2011/06/30 v3.11 Key value format for package options (HO)
pd1enc.def 2012/11/06 v6.83m Hyperref: PDFDocEncoding definition (HO)
hyperref.cfg 2002/06/06 v1.2 hyperref configuration of TeXLive
url.sty 2013/09/16 ver 3.4 Verb mode for urls, etc.
hxetex.def 2012/11/06 v6.83m Hyperref driver for XeTeX
puenc.def 2012/11/06 v6.83m Hyperref: PDF Unicode definition (HO)
stringenc.sty 2011/12/02 v1.10 Convert strings between diff. encodings (HO)
rerunfilecheck.sty 2011/04/15 v1.7 Rerun checks for auxiliary files (HO)
se-ascii-print.def 2011/12/02 v1.10 stringenc: Printable ASCII characters
beamerbaserequires.sty 2010/05/01 (rcs-revision efa082c6111d)
beamerbasecompatibility.sty 2012/05/01 (rcs-revision 67c48b3b652d)
beamerbasefont.sty 2013/10/18 (rcs-revision 72f39e01808a)
amssymb.sty 2013/01/14 v3.01 AMS font symbols
amsfonts.sty 2013/01/14 v3.01 Basic AMSFonts support
sansmathaccent.sty 2013/03/28
filehook.sty 2011/10/12 v0.5d Hooks for input files
beamerbasetranslator.sty 2010/06/11 (rcs-revision 85fd1cc7fc42)
translator.sty 2010/06/12 ver 1.10
translator-language-mappings.tex
beamerbasemisc.sty 2013/09/03 (rcs-revision a55719c41d85)
beamerbasetwoscreens.sty 2010/05/01 (rcs-revision efa082c6111d)
beamerbaseoverlay.sty 2013/12/25 (rcs-revision f6bd5e3805da)
beamerbasetitle.sty 2010/09/21 (rcs-revision f0446ed0b6ae)
beamerbasesection.sty 2013/06/07 (rcs-revision 60b9fe0f342f)
beamerbaseframe.sty 2013/10/02 (rcs-revision cdc8e9a3aaac)
beamerbaseverbatim.sty 2012/08/30 (rcs-revision dfdb135076b3)
beamerbaseframesize.sty 2011/09/12 (rcs-revision 70f9d8411e54)
beamerbaseframecomponents.sty 2013/10/18 (rcs-revision 5cf6c5555a45)
beamerbasecolor.sty 2010/06/06 (rcs-revision d1a9b48be06d)
beamerbasenotes.sty 2012/12/19 (rcs-revision 1686da3db3c9)
beamerbasetoc.sty 2013/05/23 (rcs-revision 0fdf5bc43be8)
beamerbasetemplates.sty 2010/05/01 (rcs-revision efa082c6111d)
beamerbaseauxtemplates.sty 2013/09/04 (rcs-revision 4ac715c499d0)
beamerbaseboxes.sty 2012/05/13 (rcs-revision 56972908a390)
beamerbaselocalstructure.sty 2013/09/04 (rcs-revision 4ac715c499d0)
enumerate.sty 1999/03/05 v3.00 enumerate extensions (DPC)
beamerbasenavigation.sty 2013/10/05 (rcs-revision 62be157fe783)
beamerbasetheorems.sty 2010/06/06 (rcs-revision 7e7cc5e53e9d)
amsmath.sty 2013/01/14 v2.14 AMS math features
amstext.sty 2000/06/29 v2.01
amsgen.sty 1999/11/30 v2.0
amsbsy.sty 1999/11/29 v1.2d
amsopn.sty 1999/12/14 v2.01 operator names
amsthm.sty 2004/08/06 v2.20
beamerbasethemes.sty 2010/05/01 (rcs-revision efa082c6111d)
beamerthemedefault.sty 2010/06/17 (rcs-revision d02a7cf4d8ae)
beamerfontthemedefault.sty 2012/12/19 (rcs-revision 1686da3db3c9)
beamercolorthemedefault.sty 2012/12/19 (rcs-revision 1686da3db3c9)
beamerinnerthemedefault.sty 2013/10/15 (rcs-revision 65cb471f9634)
beamerouterthemedefault.sty 2012/12/19 (rcs-revision 1686da3db3c9)
beamerthemeMarburg.sty 2010/06/17 (rcs-revision d02a7cf4d8ae)
beamercolorthemewhale.sty 2010/06/17 (rcs-revision d02a7cf4d8ae)
beamerouterthemesidebar.sty 2010/06/17 (rcs-revision d02a7cf4d8ae)
babel.sty 2008/07/08 v3.8m The Babel package
bblopts.cfg 2006/07/31 v1.0 MiKTeX 'babel' configuration
english.ldf 2005/03/30 v3.3o English support from the babel system
ngermanb.ldf 2008/07/06 v2.6n new German support from the babel system
fontenc.sty
t1enc.def 2005/09/27 v1.99g Standard LaTeX file
xltxtra.sty 2010/09/20 v0.5e Improvements for the "XeLaTeX" format
fontspec.sty 2013/05/20 v2.3c Font selection for XeLaTeX and LuaLaTeX
expl3.sty 2014/05/06 v4751 L3 programming layer (loader)
expl3-code.tex 2014/05/06 v4751 L3 programming layer
etex.sty 1998/03/26 v2.0 eTeX basic definition package (PEB)
xparse.sty 2014/05/05 v4740 L3 Experimental document command parser
fontspec-patches.sty 2013/05/20 v2.3c Font selection for XeLaTeX and LuaLaTe
X
fixltx2e.sty 2014/04/27 v1.1p fixes to LaTeX
fontspec-xetex.sty 2013/05/20 v2.3c Font selection for XeLaTeX and LuaLaTeX
fontenc.sty
eu1enc.def 2010/05/27 v0.1h Experimental Unicode font encodings
eu1lmr.fd 2009/10/30 v1.6 Font defs for Latin Modern
xunicode.sty 2011/09/09 v0.981 provides access to latin accents and many oth
er characters in Unicode lower plane
eu1lmss.fd 2009/10/30 v1.6 Font defs for Latin Modern
fontspec.cfg
realscripts.sty 2013/03/18 v0.3c Access OpenType subscripts and superscripts
metalogo.sty 2010/05/29 v0.12 Extended TeX logo macros
t3cmr.fd 2001/12/31 TIPA font definitions
nameref.sty 2012/10/27 v2.43 Cross-referencing by name of section
gettitlestring.sty 2010/12/03 v1.4 Cleanup title references (HO)
