RE: Sub Categories - The Category Menu that comes stock in the Tableau template is a one level menu. That is not a bug. If you want something with multiple sub menus you will need to install a different category menu. My guess is this was done on purpose because when on mobile you don't want to complicate things with submenus (the styling becomes quite tricky to execute.)

RE: Coloring and CSS - You will find that CSS changes on the Tableau template are rather....cumbersome. There are many style sheets, lots of use of the !important tag, a separate stylesheet.php file that seems to override actual stylesheets,etc.