So we have lots of styles, we'll start with the flat buttons. We can use the default main color theme, a ligh version, a dark version and the Bootstrap's button set.
Below there are several styles for "ghost" type or better called, lined buttons.
These buttons are a bit more special, probably to be used in some special places for some attention grabbing.
You can use several sizes for various usages.
You can use several width sizes for various usages.