It's complicated. More details in https://en.wikipedia.org/wiki/Look_and_feel

They’re copyrighted. (And you don’t have to file for a copyright to have one.)

Go ahead and reuse fonts and gradients and design something inspired by someone else’s website. But don’t copy their design in a recognizable way. The latter is both unethical and (in most places) illegal.

Depends on jurisdiction, and possibly the design, i.e. if it's very artistic it may be protected, if it's a standard theme with a few colors changed it may not be.

Are they in the same industry as you / direct competition? Be careful with this...as the last thing you want is a law suit

You can't copy distinctive branding, but there's no way to copyright a color scheme or page layout.

Maybe you can just buy a Bootstrap theme?