5
It's possible
You just have to play around the theme.html templates.
For me a first time understanding the theme system in the XOOPS took my about two days to figured it out.
For the best, you have to know the basis of CSS, HTML, PHP, and XHTML.
Also if might required some programs which can help you to edit the theme and some FTP program.
When you are making a theme, you also have to edit the templates in the administrator area. You have to Clone the Default theme before your edit.
Also in the Admin area General Setting, make sure you enable the option on theme (something for auto update theme recently). You have to turn it off if you are done with your theme.