Allow skin to be themable

[agliga]

Description

• Skin supports custom properties to override component parts

We should start to look to expose these variables in our documentation. Something like this at the end of each section

--btn-primary-background-color: --color-background-accent
--btn-primary-foreground-color: --color-foreground-on-accent
--btn-primary-foreground-color-visited: --color-foreground-on-accent
--btn-primary-destructive-background-color: --color-background-attention
--btn-primary-destructive-border-color: --color-stroke-attention       
--btn-primary-destructive-foreground-color: --color-foreground-on-attention