Change `GTK3 Theme` / `GTK Icon Theme` to `Theme` and `Icon Theme`

[eepykate]

Suggested by our favourite aquatic creature, phisch!

Also, it's inclusive of KDE users, should they choose to set it manually.

[mlvzk]

fetch.sh only detects gtk themes, changing to theme would be a lie :(

[eepykate]

it breaks search though which is important to phisch.