refactor(buttons): refactor buttons themes

[didimmova]

Closes #1995

Verify with this Handoff

[adrianptrv]

1. For the Outlined button in Bootstrap:

When we focus the outlined button in Bootstrap, the icon color remains loaded from --icon-color (blue), which makes it disappear because the background also turns blue on focus.

bootstrap-outlined.mp4

2. For the Flat button in all themes:

When we focus + click -> the border color loads from --focus-visible-border-color instead of --focus-border-color. This results in --focus-border-color not being used anywhere in the Flat button.

3. In the Indigo theme, all button types have the following behaviour (not sure if this is intended):

When we focus + click → the icon inside the button uses --icon-color-hover, while all other themes use the --focus-foreground for the icon color in that state.

When we click → the icon inside the button uses --icon-color-hover, while all other themes use the --active-foreground for the icon color in that state.