Generate SVG work for both light and dark theme

[miloyip]

Currently the output SVG grpahics has white background and black lines/text.
They look a bit odd in a dark theme rustdoc.
Hope it can use some style class of the HTML to make it fit for light and dark theme.
I am not very familiar with HTML/CSS/SVG, not sure if this is possible.

[yvt]

I absolutely agree! The trick I use in R3-OS is to detect theme changes in JavaScript code and set afilter CSS property on dark themes.

I just found that rustdoc now sets a data-theme attribute on the root element (implemented by rust-lang/rust#91062, I think), which we can reliably match by CSS rules. Maybe we could inject the necessary CSS rules to apply appropriate styling, but the exact way of doing this and whether svgbobdoc should do this is up to discussion.