Header/Nav Positioning Modification

[bodoshot]

Dear Community,

under 1.7.7 / Webspace installation /

I'm looking forward to modify my header/navigation, for my prestashop.
So far I encountered some difficulties with it I'm not able to solve.

First of all I want to switch the User-Login-Link and Cart with the Searchbar, as in the images below



Then I got the problem with the main menu, as you might already have seen. It's always aligned left and I can't figure out, how to center it.
div id="_desktop_top_menu"
text-align:center; align-items:center, padding-left..., have all not worked so far.

I hope and know ya'll are able to help me out
Looking forward to some helpful replies.
Thanks in advance.