Add dark mode

[janfaracik]

Fixes #6222

This MR adds a dark theme for the site, following the user's system preference. I've widened our usage of CSS variables throughout the codebase so that colours should automatically adapt, I may have missed some so do let me know if you spot something that looks off.

I've went through the majority of pages and they're looking correct, I've highlighted current issues below:

Potential issues

• Logos on the homepage are hard to read in dark mode Had to recreate the OSL logo by hand as I couldn't find an available SVG online - happy to contact them to see if they have one however
• Cloud providers cards are a little low contrast in dark mode

[zbynek]

Fixes #6222

Some testing needed for all the sites that reuse jenkins.io styles (accounts, plugins, updates, stories)

[kmartens27]

dark mode looks really good on jenkins.io and appears to be consistent through the site.

[janfaracik]

Raised jenkins-infra/jenkins-io-components#146 to look into how we can share CSS vars across the various Jenkins sites - open to any ideas.

[zbynek]

• Accounts app will need to load styles.css to benefit from this, currently only uses jenkins.css
• Stories have minor issues (see navigation and heading in https://stories.jenkins.io/user-story/to-improve-railway-signaling-solutions, the image on the front page should have transparent background with outline).
• Updates site, plugins looks OK in general, one thing that I'm not sure about is the color of inline code: usually this shade of red indicates error.