Consolidate location of dark mode rule overrides

What does this MR do and why?

Move dark mode rule overrides into a single file.

These were split between two files before (some in _dark.scss and some in application_dark.scss, making them harder to manage, and occasionally meaning they were overridden by app styles. This is a simple way to fix that.

Changes output CSS order slightly, no UI changes are expected (in terms of what to look for, main changes are around navbar & gl-labels in dark mode).

Split off from this other thing