Adjust colors of AvalonEdit built-in highlightings for dark themes

[ltrzesniewski]

Fixes #3137

Problem

Built-in highlightings of AvalonEdit are meant to be used on a white background, and look wrong on a dark one.

Solution

• Any comments on the approach taken, its consistency with surrounding code, etc.

  This PR changes the colors provided by a highlighting, unless that highlighting comes from ILSpy itself.

  The color is tweaked in the HSL color space, inverting the luminosity and desaturating the color a bit (as recommended by Material Design). A few coefficients are applied, I chose them after trying various solutions, as I couldn't find a known algorithm for switching a color for use in dark mode anywhere.

• Which part of this PR is most in need of attention/improvement?

  I basically made up the color adjustment function out of nowhere. If you know something better, I'd like to know.

• At least one test covering the code changed

  This only contains UI changes.

Here are a few examples:

PowerShell:

JSON:

Turns out, HTML is handled by ILSpy's highlighting for XML.

[siegfriedpammer]

Thank you for fixing this!