Rename GResources for standard GTK themes

Rename GResources for standard GTK themes

After commit 2ab9be54fb34c61eedbbb89b78ce9db23ba81e8a we had to rename
the generated CSS files to be included into the GResource bundle; we
kept the URI stable to avoid too much churn, and allow backporting the
change to the 3.24 stable branch. This had the adverse effect of making
it harder to debug issues, as the on-disk file name does not match the
location in the GResource that will be used to print out warnings,
errors, and debugging messages.