Improve consistency in header's layout and size

[MaikelChan]

Hi!

I just fixed some inconsistencies in the header:

• In mobile version, its height would depend on the height of the dark mode switcher. If the user has the switcher disabled, the header would appear very small on mobile.
• Dark mode switcher is smaller on desktop mode than on mobile, and compared to the other icons.
• Dark mode switcher on mobile doesn't have color change on hover, the other buttons have.
• Clickable area in dark mode switcher is much bigger than the other icons (that's why the height of the header in mobile depended on this button).

[MaikelChan]

I also added hover color to the header menu and nested menus, the same as the search and dark mode switcher buttons. They didn't have any reaction to being hovered.