To install a new theme on your community, go to Admin Cpanel > Customization > Themes. At the top right, select the + Add Theme button.
From the popup box, choose the Upload Theme tab and select the .xml file from your computer.
Once it has processed, you'll be returned to the Theme list. To set the theme as default, click the Edit button and enable the Default theme? switch. To make the theme visible for all members (suggested in most cases), select the All box under the Available for list.
Save and you're done!
IMPORTANT: Version 4.x has a "feature" which attempts to inherit the theme and apply it to your ACP. In most cases, this may impact visibility or your layout. To use the Default theme in your ACP, go to ACP > "Signed in as..." > AdminCP Theme > Default.