Hi bernardlefebvre
Correct procedure to install a new uber theme is as follow
1) After first quickstart installation of template you should go to Extensions - -> Manage -> Install -> and choose from Tabs the Joomlart installer.
2) Select your theme and then install it -
3) After you installed your desired them the plugin - Joomlart installer will be automatically disabled
So if you want to install a new Uber theme you first should enable again the joomlart installer plugin and repeat step from point 1
P.S.
Each time you install a theme it will be like a new installation , so everything was installed before on your website will be deleted - So if you have RS-Firewall it should be installed again at the end of theme installation procedure.
Hope it clarified your questions.
Regards