Dark mode fixes

[pokonski]

Description

Followup PR for #2562 with fixes to dark mode styling, mostly in Examples:

• missing btn-danger class fixed
• corrected usage of btn-primary to primary class
• added dark mode styles for buttons in general
• applied btn primary/btn danger classes to buttons that had no styling until now
• added dark mode styles for the server requests bottom drawer in Examples
• fixed styles for tabs examples in dark mode

Corresponding issue:

Testing

Locally via Zola

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded