Tikz and enveloping border

Question

How can I add an enveloping border around the blue line in this MWE?

\documentclass[tikz,border=5mm]{standalone}
\begin{document}
\begin{tikzpicture}
\draw[blue, line width=10pt](0,0)--(90:1.5)--(60:2.5)--(30:3.0); 
\end{tikzpicture}
\end{document}

PS. I found out that something similar is possible with MetaPost in this previous answer https://tex.stackexchange.com/a/330016, but I'd like to know if this is also possible with Tikz.

Answer 1

This is Salim Bou solution with negative shorten values and declared as a style which adds a preaction.

Style border has three parameters:lower line width, border width (applied to shorten) and color.

\documentclass[tikz,border=5mm]{standalone}
\begin{document}
\begin{tikzpicture}[border/.style n args={3}{
    preaction={draw=#3, line width=#1, shorten <=-#2, shorten >=-#2}}]
\draw[blue, line width=10pt, border={14pt}{2pt}{green}](0,0)--(90:1.5)--(60:2.5)--(30:3.0); 

\draw[blue, line width=10pt, border={14pt}{2pt}{green}](3,0)rectangle++(2,3); 

\draw[blue, line width=10pt, border={14pt}{2pt}{green}] (7,1) circle(1cm);

\end{tikzpicture}
\end{document}

Answer 2

A solution based on this answer. This answer comes with an Outline style, which has one argument, the distance from the center to the line to the contour. The width is then twice this length.

\documentclass[tikz,border=5pt]{standalone}
\usetikzlibrary{decorations}

% based on https://tex.stackexchange.com/a/103088/121799
\def\pgfdecoratedcontourdistance{0pt}

\pgfkeys{/pgf/decoration/contour distance/.code={%
    \pgfmathparse{#1}%
    \let\pgfdecoratedcontourdistance=\pgfmathresult},%
    /pgf/decoration/contour name/.store in=\ContourName,
    /pgf/decoration/contour name=mycontour
}

\pgfdeclaredecoration{contour lineto}{start}
{
    \state{start}[next state=draw, width=0pt]{
    \pgfcoordinate{\ContourName-0}{\pgfpoint{0pt}{\pgfdecoratedcontourdistance}}
        \pgfpathlineto{\pgfpoint{0pt}{\pgfdecoratedcontourdistance}}%
    }
    \state{draw}[next state=draw, width=\pgfdecoratedinputsegmentlength]{       
        \pgfmathparse{-\pgfdecoratedcontourdistance*cot(-\pgfdecoratedangletonextinputsegment/2+90)}%
        \let\shorten=\pgfmathresult%
        \pgfpathlineto{\pgfpoint{\pgfdecoratedinputsegmentlength+\shorten}{\pgfdecoratedcontourdistance}}%  
    %\stepcounter{Outline}
    \pgfcoordinate{\ContourName-1}{\pgfpoint{\pgfdecoratedinputsegmentlength+\shorten}{\pgfdecoratedcontourdistance}}
    }
    \state{final}{
        \pgfpathlineto{\pgfpoint{\pgfdecoratedinputsegmentlength}{\pgfdecoratedcontourdistance}}%
        \pgfpathlineto{\pgfpoint{\pgfdecoratedinputsegmentlength}{0pt}}
    }   
}
\tikzset{Outline/.style={ postaction={
        decoration={contour lineto, contour distance=-#1,contour name=mycontourA},draw=blue,
         decorate},
        postaction={
        decoration={contour lineto, contour distance=#1,contour name=mycontourB},draw=blue,
         decorate},}}
\begin{document}

\begin{tikzpicture}
\draw[red,line width=10pt](0,0)--(90:1.5)--(60:2.5)--(30:3.0); 
\path[blue,very thick,Outline=5pt](0,0)--(90:1.5)--(60:2.5)--(30:3.0); 

\begin{scope}[xshift=4cm]
\path[blue,line width=2pt,Outline=5pt](0,0)--(90:1.5)--(60:2.5)--(30:3.0); 
\end{scope}
\begin{scope}[xshift=8cm,scale=2]
\path[blue,line width=4pt,Outline=10pt](0,0)--(90:1.5)--(60:2.5)--(30:3.0); 
\end{scope}
\end{tikzpicture}
\end{document}

NOTE: This works only with polygons which are not closed.