Issues regarding the theme toggle buttons in the header

[ivarnakken]

webkom/lego-webapp#2980 introduced buttons to toggle the theme in the header. Here's a collection of issues I have found regarding the feature. Any contribution fixing any of the issues below is much appreciated (know that you don't have to fix everything with one single PR).

• When changing the theme through the dropdown, the other icon in the header is not updated accordingly. See image below for illustration.

• When spamming the button (especially the upper one), it'll create a queue of theme changes, which to the user seems like the page is glitching (because it'll toggle between the different states after stopping the spamming).

• The icon in the dropdown should change color when hovering over the entire row, and not just icon itself.

• On the upper icon, the space that is clickable and the space that triggers the color change, are not the same.

• The default color of the upper icons do not match, which is more noticeable on dark theme. See image below for illustration.

• The text in the dropdown, "Endre tema", has a slightly bolder styling, and should preferably match the other rows, despite having a different color. See image above for illustration.

• When changing the theme, the fancy nodes in the background will update. I think it would be better if it didn't - less stuff happening.