Improve differentiation between selected and unselected "selectable buttons"

[jcogs-design]

Is your feature request related to a problem? Please describe.
Currently it is difficult to see whether a selectable button has been selected or not...
Here is an example:

Describe the solution you'd like
Revision to the button CSS to make the selected status more strongly differentiated from the unselected state.

Describe alternatives you've considered
Using some other control... but would prefer not to.

Teachability, Documentation, Adoption, Migration Strategy
No wider implications

• I am capable and would like to work on implementation of this feature if it is considered.

[intoeetive]

@ajohnson FYI

[ajohnson]

This is currently using EE's default active button state for selected buttons. But we can probably add extra emphasis to that state targeted at just this fieldtype. I'll put this in the Core team's queue. Thanks!