Import the theme files into a subdirectory without any changes, even not
removing the trailing whitespace, to facilitate updating them in the
future. Do not add the theme as a submodule as it's surprisingly big
(~12MB) and we don't want to spend extra time on cloning it in each of
our CI builds.
Using this theme gives a nicer appearance and supports the often
requested dark mode.
Remove all colour-related options from the existing custom CSS file as
they don't work well in dark mode.
Also switch to using SVGs with transparent background, rather than PNGs,
for the class diagrams, to avoid background colour mismatch in dark
mode.
