Hi there,
1)
a) You can download themes from
Zimbra :: Gallery - search for the newer ones to ensure they are compatible.
Older themes may be incompatible, or may lack icons for newer features.
b) Unzip the .zip file you have downloaded.
c) Open a file manager with root permissions (in Gnome Nautilus, in KDE Dolphin). To do this go to terminal and enter 'sudo nautilus' or 'kdesudo dolphin', followed by your root password.
d) In the file browser window that opens, find the unzipped folder for the theme you have downloaded. (NB Be cautious because you are using root in the file browser..)
e) Copy it and paste in in the following folder: /opt/zimbra/zdesktop/jetty/webapps/zimbra/skins/
f) Close the file brower.
g) Close and reopen Zimbra Desktop.
h) Go to Preferences in Zimbra Desktop and under 'All accounts' > 'General' > 'Login options' you can select the new theme (or go back to the old theme).
i) Ensure you click 'save' in preferences.
i) A popup window will ask you to reload Zimbra, accept, and Zimbra Desktop should reload with your new theme.
OR
a) Vote for this bug to be implemented:
Bug 56978 Allow user to add/remove Themes with Gallery intergration