[$10] - Fix styling issues in the top nav menu

[birdofpreyru]

Both screenshots are from the pages served by code from this repo. Both are unforthseen side-effects of other changes.

1. User menu in the Mobile view ovelaps with other menus.
   
   You'll see, this problem is introduced by my change in styling, done in the commit 801c118. The reason to do that change was that without it, in the Desktop view, the user menu, when collapsed, is a bit higher, than the containing menu bar, which leads to a small undesirable spacing between the menu bar and the page content, which in turns is a problem for our new challenge listing. So, please fix it in a way, that menu is properly styled for Mobile, though does not re-introduce that problem for the new listing and other pages.

2. Same menu, open in the Desktop view. Note that there is a small spacing between bluish highlight of active/hovered menu tiles and the top of the menu. Should be removed.