T3 framework uses LESS to compile the CSS being used by the template. LESS is recompile for a multitude of reasons and it sounds like installing a new extension is one of them. When LESS is recompiled, you will lose any edits you have made to the CSS.
In order to made CSS edits that are not overwritten by LESS you have to put any custom CSS in custom.css. The template is configured not to touch that file so your changes are not lost.
Documentation is here - http://t3-framework.org/documentation/customization.html#custom-css