[docs] Adjust Base demos for dark mode

[michaldudak]

As pointed out in #30113 (comment), some of the unstyled demos don't look right under dark mode. This issue is meant to track work related to updating all the unstyled demos so they look great in both light and dark modes.

[danilo-leal]

Hey! Does it make sense to have an umbrella issue for this task? I think we should have the demos looking great as part of any component PR, not merging it before the documentation for it is good enough. As far as I can see, Select is the only one right now in need of a little tweak (maybe a bit the inactive Switch as well) on dark mode but all the other ones are mostly fine. Do you have anything else in mind?

[michaldudak]

Let's update Select and Switch demos as a part of this issue then and make sure in the future that dark mode demos look well before merging, as you suggest.