Create a new file named custom.css in the htdocssitenametemplatestemplatenamecss folder.
Insert your css in that new file. Then see what happens.
T3 runs on LESS. T3 “styles” content by compiling LESS into CSS. Each time it compiles, all CSS files are overwritten except the custom.css file. So editing any other css file is fruitless because it will be overwritten.
If you can accomplish your goals with custom.css, then that is all you need. If not, then work in the LESS files.
Read about Development Mode and what it entails. http://www.t3-framework.org/documentation.html
Good luck!