Uploading a custom CSS style sheet is fairly simple.  If you have a FTP program and know how to use it you can browse to Data/sites/[site number]/skins/[skin you want to add file to] and copy/past the file into the skin folder.  Then open up the style.config file in the same skin and add <file>[name of your style sheet].css</file>underneath all the other entries there, but make sure it's above </cssfiles>.  Your styles should be included now, but you may have to clear your server cache to see the results. Read this to find out how that is done https://www.mojoportal.com/skinning-overview.aspx.

HTH,

Elijah