Add support for local them development in the demo + Allow developing the dark login theme in the demo repo

[oleg-nenashev]

I have started working on a Dark Login theme PoC. Idea:

• - Rework Jenkins Core CSS login theme files to use variables. From what I see in the code @scherler actually started doing it in some files, but then commented it out
• - Introduce a new CSS file with light defaults in the Jenkins core. Variable names should be aligned with Add CSS variables - prerequisite for Dark mode  jenkins#4752 to simplify overrides. [JENKINS-62515] - Introduce CSS variables in the Login screen CSS jenkins#4763
• - If needed, Update Dark Theme CSS to include overrides for Login Theme CSS variables
• - Document how to add a custom stylesheet to the login theme

Current PoC works for the background, but more work is needed to make it actually working. We will not be taking the CSS files from the core into the final implementation, of course

[oleg-nenashev]

Moved the Core CSS patches to jenkinsci/jenkins#4763

[timja]

this needed?