CI: a new style for LCOV reports

The standard style of LCOV HTML reports is a bit harsh to look at. This commit replaces it with a new one.

The new CSS was written from scratch by looking at the HTML source code of a generated LCOV report. The original gcov.css file was not used.

The color scheme is neutral, trying to avoid a Christmas tree effect. The colors are intended to be calm while also distinguishable even for colorblind people, and not hamper text readability.

The font lists were taken from Gitlab with the hope that it will blend in a little better when viewing from MR artifacts.

I'm not sure how well I actually reached those goals, but here goes. Let the bikeshed painting begin. ;-)