Hi Angelo,
The css files of template are compiled from .LESS files so it's not good to customize the files as the things will be lost once you compile again. To improve your site performance, you can optimize the .css files. In the T3 BS3 Blank template style manager, open the General setting panel, disable development mode and enable CSS & JS optimization, check the screenshot below:
http://easycaptures.com/fs/uploaded/873/2327421263.png
Regards.