I can’t find a way to edit the mobile CSS for SocialEngine 4.2.3 (I had the file mobile.css in application/themes/default when I installed). I can’t find it in Layout -> Theme Editor.
edit Forgot to add, for those who don’t know, if I edit the file myself with an IDE, it doesn’t update in SocialEngine’s pages.

The mobile.css is not available for edit in the Theme Editor. You can edit the mobile.css in {site root}/application/themes/{your current theme}/mobile.css. Keep in mind that that different modules also have their own CSS. They are usually located in {site root}/application/modules/{the module whose CSS you’d like to edit}/externals/styles/mobile.css.