Fix import config alert

[IraSoro]

Closed #270

I fixed this problem.
But it will work only if: you already have a dark theme, you import a config with a dark theme, the alert is displayed with a dark theme. But if you import a config with a light theme, the alert will be displayed with a dark theme this time.

The dark alert looks like this:

I also removed the word beta from the dark theme selection.

And changed icon for theme select. The new icon looks like this