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/slider.go

@@ -21,7 +21,7 @@ func Slider(th *Theme, float *widget.Float, min, max float32) SliderStyle {
 	return SliderStyle{
 		Min:   min,
 		Max:   max,
-		Color: th.Color.Primary,
+		Color: th.Palette.ContrastBg,
 		Float: float,
 	}
 }