Separating color palette from Theme

[cylewitruk]

I think it would be a nice user-friendly feature to be able to specify color palette(s) without having to override the Theme and custom-implement the bar_color() and state_symbol_color() methods. This could be something like:

pub struct ColorPalette {
    // Result colors
    pub success: Rgb,
    pub error: Rgb,
    pub warning: Rgb,
    pub info: Rgb,
    pub remark: Rgb,
    
    // State colors
    pub cancel: Rgb,
    pub inactive: Rgb,
    pub active: Rgb
}

pub struct ThemeColors {
     pub light: ColorPalette,
     pub dark: ColorPalette
}

Where the user could provide their own ThemeColors (stored in a Lazy next to the Theme), and we use something like termbg to determine if the light or dark palette should be used.

Note that I've expanded on the terminology above -- I think the submit state is a little confusing, it's not really clear to a user what is "green", "cyan", etc., so I'm also suggesting revisiting the naming to more "conventional names" that are associated with certain colors.

Thoughts?