[do not merge] Switch pages theme

[MaggieSalak]

Changes the theme to dark cayman using the modified theme from https://github.com/observability-lab-cse/lab-theme
Tested the theme and it looks ok; every section still shows the title 'Observability Workshop' as expected

[MahrRah]

Lets maybe check if everything is correct in the custom theme before we merge :) But else looks good!

[MaggieSalak]

@MahrRah I managed to accomplish what we wanted without using our custom theme. Instead, I changed to use the dark cayman theme we wanted and just added a custom layout to make sure we always show the site title instead of the page title :)

[MahrRah]

Nice! I just clicked through it one thing I notices is that the drop downs are not as visible any more. Ben had the css style for those classes updated to have a background and the arrow i think. I think we should fix that as well, just to be sure they are not overlooked.

[MaggieSalak]

@MahrRah indeed there is a problem with dropdowns. I tried making this work but couldn't. At this point I've spent so much time on this that I'd say we have other lower hanging fruits and more important tasks than changing the theme :D. Unless you have an idea how to make the dropdown work, then feel free to update of course.