Complete rework of git CSS
The over-complicated color hierarchy of default cgit CSS is no more.
The navigation tab bar is simplified to plain text with brackets and
margins separating the page links. While not being responsive, this does
declutter the web page significantly.
Section headers are consistently styled like <h2>'s throughout (such as
repo section headers on the index page, and commit subject lines on
commit pages). For now I am leaving table header rows as-is, but I may
go through all pages and edit their appearance as well.
The commit/diff pages have been reworked. The diff options, diff stats,
commit metadata, and commit message components of the page are now shown
as panels with a distinct background and padding.
The colors for the diff page have been reconfigured entirely. As well,
related colors (such as the diff stats bar charts, or the log page
insertions/deletions counts) are brought in line with these colors.
More generally, all colors are now set by CSS variables.
The branch/tag decorations (on the summary, tree, commit, and diff
pages) now are simplified and appear as highlighted text instead of
floating buttons.
The log and plain links on the tree page are finally fixed.