Move jQuery UI theme customization to a separate SASS file

[#44817219]

Now it should be clear where general jQuery UI styles modifications
should be placed. We can treat it as our "jQuery UI Lab theme".

I was also trying to use ThemeRoller, but its features are not so
wide. So basic theme + custom modifications should be simpler and more
flexible.
Cleanup _application.sass, rm styling of old panes

[#44817219]
Add section about known problems with build on Linux in readme