Video: https://www.screencast.com/t/jhMrG1XW
Log in as Admin
Make sure, that in "User profile: Zabbix Administrator" settings you
have chosen and submitted "System default color theme".
Then go to Administration - General (GUI)
Change Default theme from Blue to Dark.
Press Update.
Observe that color theme is not changed.
Then change Dark theme to High-contrast light.
Press Update.
Observe that color theme is changed to previosly selected - Dark.
Then change Dark theme to High-contrast dark.
And observe that color theme is changed to previosly selected - High-contrast light.
And so on.

This happens, because page is not refreshing right away after submittig the form.

Expected Result:
Success message "Configuration updated" appears AND page refresh performes and color theme changes.

The second problem is that if in user settings "User profile: Administrator" is set any other theme besides "System default" it overrides Administration - General (GUI) settings. So no matter what you choose there in Default theme dropdown, color theme won't change even after refresh and no info message will appear.
Video: https://www.screencast.com/t/c6FCDzNHpm

Expected Result: Add info message near Default theme dropdown:

• This setting applies for all users, but is overridden by individual User profile settings.