How do I apply a theme in Linux?

If you’d rather install the theme system-wide so everyone can use it, place the theme folder in /usr/share/themes. Open your desktop environment’s settings. Look for the Appearance or Themes option. If you’re on GNOME, you’ll need to install gnome-tweak-tool.

Where is theme folder in Ubuntu?

Default themes directory is /usr/share/themes/ but it’s editable only for root. If You want to edit themes the default directory for current user would be ~/.