Theme stylesheets not loaded I am learning Drupal 8 and have installed Drupal 8.1.2 version on XAMP. 
No theme files are included when page is loaded (viewed source).

When trying to solve this, I changed theme from default bartik to stark. Still no theme files are loaded. 
And now I am trying to switch it back to bartik, but I am unable to.

"Bartik is now the default theme." is displayed but Stark 8.1.2 (default theme) is default theme.

See screenshot below:
I recommend you use Aquia Dev Desktop, easier to use than Xamp, one click install.

https://dev.acquia.com/sites/default/files/downloads/dev-desktop/AcquiaDevDesktop-2-2016-09-05.exe

Just use that, and it will all work 100% the first time.

Something else you might need to do with Xamp is adjust timeouts ect...with Aquia dev desktop the settings are usually already adapted to Drupal requirements.