Just started exploring these.

Found via Tools | Extensions

or Tools | Themes

You can see what is available via "Get More ..." in the lower right corner.

My favorites so far are the Extension: Dictionary Search
which adds a right click direct dictionary search. highlight the word & look it up.

& my fav theme: phoenity

Silver Skin theme is pretty cool!

I also like the Auto Copy extension

I have just downloaded the SphereGnome theme. I like it.