I remember changing from breath to breath-dark, but nothing else. Now my user name in the menu is in black, is there some way to fix this?
I can’t embed images nor add links to my post somehow. So I can’t show the problem.
Hi and welcome to the forum, 
New users are limited in ability on the forum to prevent misuse.
You can post the image to a host you control or a public image hoster and leave the link like this:
https://somehost.somewhere/file.extention using the backtick `lke this:
`https://somehost.somewhere/file.extention`
or remove the dots in the link
After changing themes, logging out and back in may be required for all changes to take effect. If only the application menu did not update, press ALT+F2 and type:
plasmashell --replace
The screen may flicker momentarily while the desktop shell restarts.
Not to forget Ctrl+Alt+Del to quit the session and log in again, but often I just do a reboot after messing with themes and stuff to check the login screens.