In five simple steps:
1) In the "templates" directory, copy the "tmpl_uni" folder under a different name.
Example: tmpl_newskin
2) Update the file below to set the name of your new template
File: ./templates/tmpl_newskin/scripts/BxTemplName.php
Example: $sTemplName = 'newskin';
NOTE: If you skip this step then your new template won't appear correctly in the Advanced settings menu (see step 4)
3) Update the files in the "tmpl_newskin" folder the way you want (html, css etc...)
4) Log into the admin section and go to Settings -> Advanced settings
Near the bottom of the page you'll see a "Template" option with a dropdown menu containing the various templates available (set to "UNI" by default)
NOTE: If you skipped step 2 then all you'll get in the dropdown menu is "UNI","UNI",... that's because the <option> value is set correctly to "newskin" but the <option> label still says "UNI" (the default value of the $sTemplName value in BxTemplName.php). Bit clumsy but anyway...
5) Select your new template ("newskin" in this example) then click on the "Save changes" button
Et voila, you should now be done and ready to go with your new look & feel!
./scripts/BxTemplConfig.php
./css/general.css
im scratchin my head here, then see more
Depending upon who created the template you're trying to use, some of them are not as reliable as others... try to look into the files listed above and make sure the variuous variables are set properly. Also, my post is almost 1 year old and only relates to Dolphin 6.1.4 - so not sure how things evolved since then in terms of template management/creation...