You simply have to upload the new theme to the "themes" folder.
Now enter your database over phpMyAdmin or the backend.
Enter the site_config table and watch out "id 9", the availabelValues should contain:
["blue_v2"] or something like that.
Simply change it to:
YetiShare sadly does not really have much of a "theme/style" system set up. The "themes" folder only includes images and css. Ultimately, you can over write the blue_v2 theme without having t go into the database to make anychanged.
Anyhow, any changes you would like to make to the theme itself may require more then just css changes. such as extensivly changing the look of the site rather than just the "style"/color.
With that, you would need to edit most of the core files such as the index, _header, _sidebar, _footer, etc. All of which if you wish to create a theme that looks nothing like the yetishare script will be needed to be edited.