From MEPIS Documentation Wiki
Simply download the theme or icon package --usually a compressed tar.gz file type-- to your home directory. A good site for getting themes/icons is www.kde-look.org/
No need to extract the archive to install it. Just open the KDE Control Centre (kcontrol at a command prompt), click on 'Appearance & Themes', then select either 'Icons' or 'Theme Manager', and finally click 'Install New Theme'. It's the same process for icons and themes. Just browse to the downloaded theme/icon package, select it and click OK. To use the new theme or icons, select the newly added theme or icons from the list and click 'Apply'. Easy!
Note: There may be some themes that have a different install method. Read the release notes from the download page to be sure.
Customizing Themes in KDE 4 (MEPIS 8.5 and beyond)
- Open System Settings
- Goto Advanced tab >>> Desktop Theme Details.
- Change the elements you want to change so that the "Apply" button is not greyed out.
- Click "Apply" (now it is not greyed out) - and you get a dialogue stating : "To change your desktop theme to "(Customized)", open the desktop Appearance Settings and select "(Customized)" from the droplist. "
- So now: right click the desktop and choose Folder View Settings
- In the desktop theme dropdown box, choose "customised" and click Apply