Theming Quality of Life Improvements

[mad-moo]

• Default Rectangle.fill to transparent
• Allow passing in text style parameters like the color and font weight to a rio.Text without having to create a rio.TextStyle
• Consistently name palettes the same thing. i.e. ThemeContextSwitcher -> PaletteSwitcher. ColorSet -> Palette. Also allow passing rio.Palette instances to components looking for a ColorSet

[mad-moo]

Also, fix this inconsistency: TextStyle uses None to indicate keeping the default text color, but other components use different values. For example rio.Icon uses "keep"