[Feat]: change style for disabled buttons, etc.

[rjamesnw]

Is your feature request related to a problem? Please describe.
My app background is black. When I disable a button the icon goes dark, and it's almost invisible on a dark background, and there's no way to change it in the component settings.

I have the same issue for the placeholder of an input. The background is transparent, showing the black background, and the placeholder text is too dark to see.

Describe the solution you'd like
Add a style section for disable state to change the disable colors and input placeholder text.

Describe alternatives you've considered
CSS hacks if need be, or by using JS hacks (I'm assuming this may work).

Additional context
Using v2.4.12-"dev #5e6cf42" on Docker for Windows.