LaTouf wrote:
For example, I want my main title to be centered in a rectangular block with some bg color, whose geometry is fully defined by me. Is that possible ?
It is possible. The place you want to look is in the beameroutertheme<themename>.sty file. You probably want to create your own based on one that is included. Then you want to modify part that starts with
\defbeamertemplate*{frametitle}
I am attaching my hacked version of the sidebar outer theme. To try it out, you will want to put
\useoutertheme{mysidebar} in your preamble
beamerouterthememysidebar.sty