Hello Hodor!

Alex answered how to proper enable RTL in your site and add own rtl.css file to template. But common.css file is the system css file so it's strongly recommend not to rename / remove it. You may redeclare rules from there but in your own file (common.css for example in new template are fine too). Here is the list of system CSS files which should be in UNA site always:

'common.css',
'default.less',
'general.css',
'icons.css',
'colors.css',
'forms.css',
'media-desktop.css',
'media-tablet.css',
'media-phone.css',
'media-print.css',

With the best regards, Leonid