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, 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.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>