Fix theme toggle button bugs

[Bestem0r]

Description

• Adds background hover effect to the theme change button in the drop down menu.
• Fixes clicking on the toggle theme button in the header not changing the icon of the button in the drop down and vice versa.
• Adds debounce to theme changing to prevent "glitchy" behaviour.

Result

Before	After
Without background hover	With background hover
Not synchronised icons	Synchronised icons

Testing

• I have thoroughly tested my changes.

1. Click on your profile picture to open the drop down menu.
2. Hover over the change theme button and verify that there's a hover effect present.
3. Click on the change theme button and verify that the icon in the header also changes.
4. Spam click the header change theme button and verify that the site doesn't change theme after spamming has ended.

Resolves: #2926

[LudvigHz]

Hi! Thanks for your contribution! ⭐

Just move the eventListener to the useEffect and I'm happy to merge this in!

[LudvigHz]

Awesome!

The CI is just failing on lint (you're missing a newline in the css 🙃) so when CI passes this can go in 👍

[eikhr]

Awesome! Just come by the webkom-office in F252 if you want a LEGO-pin:) There is almost always someone there

[ivarnakken]

Thanks a lot! 🚀🚀

Looks nice, but I have a small nitpick 😅

[itsisak]

Awesome!💯

[LudvigHz]

@Bestem0r Another slight issue here 😅. We had a small issue with the CI that was fixed a few days ago, so you need to rebase this on master for CI to work properly.

[falbru]

LGTM 🚀 Remember to squash before you merge!