message: Introduce `Custom` style variant for `Rule`
diff: @@ -284,17 +284,32 @@ impl pane_grid::StyleSheet for Theme {
  /*
  * Rule
  */
  +#[derive(Clone, Copy)]
  +pub enum Rule {
  + Default,
  + Custom(fn(&Theme) -> rule::Appearance),
  +}
  +
  +impl Default for Rule {
  + fn default() -> Self {
  + Self::Default
  + }
  +}
  +
  impl rule::StyleSheet for Theme {
  - type Style = ();
  + type Style = Rule;
  - fn style(&self, _style: Self::Style) -> rule::Appearance {
  + fn style(&self, style: Self::Style) -> rule::Appearance {
  let palette = self.extended_palette();
  - rule::Appearance {
  + match style {
  + Rule::Default => rule::Appearance {
  color: palette.background.strong.color,
  width: 1,
  radius: 0.0,
  fill_mode: rule::FillMode::Full,
  + },
  + Rule::Custom(f) => f(self),
  }
  }
  }
  