widget/material: manage widget colors with Palette type

This introduces a new material.Palette type that captures the color information
necessary to render a widget. This type is embedded in the material.Theme to
make it easier to swap to a different palette for part of the UI by reassinging
the Palette field.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/material/loader.go

@@ -22,7 +22,7 @@ type LoaderStyle struct {
 
 func Loader(th *Theme) LoaderStyle {
 	return LoaderStyle{
-		Color: th.Color.Primary,
+		Color: th.Palette.ContrastBg,
 	}
 }