I could put the following Tikz code on a single page:
\tikz[remember picture,overlay]
\shade[top color=green!30, bottom color=blue!30]
(current page.north west) rectangle (current page.south east);
However, it fails if I try to use it as a background for all pages.
I was also unsure whether to use overlays or layers -- tried both (see below)
\usemodule[t-tikz]
\usetikzlibrary[shapes.geometric,arrows,arrows.meta,positioning,mindmap,shadows,trees,calc]
\defineoverlay[sombra][
\tikz[remember picture,overlay]
\shade[top color=green!30, bottom color=blue!30]
(current page.north west) rectangle (current page.south east);
]
%\setupbackgrounds[page][background=sombra]
\setlayer[mybg] % name of the layer
%[hoffset=1cm, voffset=1cm] % placement (from upper left corner of the layer)
{\tikz[remember picture,overlay]
\shade[top color=green!30, bottom color=blue!30]
(current page.north west) rectangle (current page.south east);
} % the actual contents of the layer
\setupbackgrounds[page][background=mybg,
state=repeat % repeat each page
]
\starttext
asdf asdf
\stoptext
Edit
I was able to get it working with this -- though I've got to fix this white part with some offset I couldn't figure out so far.
\setuppagenumbering
[alternative=doublesided]
\setupcolors
[state=start]
\definelayer
[fundo]
[repeat=yes,
width=\paperwidth,
height=\paperheight]
\usemodule[t-tikz]
\usetikzlibrary[shapes.geometric,arrows,arrows.meta,positioning,mindmap,shadows,trees,calc]
\setlayer
[fundo]
[preset=middle]
{\tikz[remember picture,overlay]
\shade[top color=green!30, bottom color=blue!30](current page.north west) rectangle (current page.south east);}
\setupbackgrounds
[page]
[background=fundo]
\starttext
asdf
\page
asdf
\stoptext
layer
oroverlay
?context --luatex <filename>
. If it's helpful, I'll post a solution using MetaPost