Problems changing themes

I have been having a tough time changing my themes. Firstly I do not see any theme apart from – Twenty Fourteen, Twenty Fifteen, Twenty Sixteen and Radcliffe(the theme I had selected during registering).
I downloaded Tracks from WordPress Themes and tried to upload that, but is says –

“An unexpected error occurred. Something may be wrong with WordPress.org or this server’s configuration. If you continue to have problems, please try the support forums.”

I tried uploading it via FTP, but faced a CSS missing notification(I tried both .zip and unzipped uploads). Anyone who can help me out with this? I am new to the system and I’ve tried out basically all the methods available.