yep, restarting the browser did the trick
but changing the theme for a specific user didnt work so far. what i did so far:
* change user-theme for user XY
* logged in with user XY (via the login-button, right to the user in the system-user-list)
* restarted browser
the user gets displayed the default-theme
as i only need 1 theme, im changing the default-theme now (so it works for me)