feat: add support for dark / light mode switching (#178)

* feat: add support for dark / light mode switching and simultaneouscustom light / dark mode themes

* refactor(color-picker): optional initial color and fallback color

* refactor: used FixedPortion for layout of the settings item

This makes sure that the control always has at least the specified portion of the available space

* refactor: make all members of the ThemeBuilder public

* refactor: add and update palette colors

* fix(theme): typo and derive PartialEq for ThemeBuilder

* fix: update color picker usage

* feat: add more variables to the theme

* fix: radius on headerbar

* fix: Theme CosmicConfigEntry impl

* chore: specify rev of taffy

* fix: theme CosmicConfigEntry missing variables

* fix: apply theme type when theme mode changes

* wip: add plus icon to empty color picker button

* chore: fix rev and imports

* refactor(color-picker): allow custom size for the icon

* refactor(color_picker): make color_button public

* update iced

cosmic-theme/src/model/mode.rs

@@ -0,0 +1,46 @@
+use cosmic_config::{Config, ConfigGet, ConfigSet, CosmicConfigEntry};
+
+/// ID for the ThemeMode config
+pub const THEME_MODE_ID: &str = "com.system76.CosmicTheme.Mode";
+
+/// The config for cosmic theme dark / light settings
+#[derive(
+    Debug, Clone, Copy, PartialEq, Eq, cosmic_config::cosmic_config_derive::CosmicConfigEntry,
+)]
+pub struct ThemeMode {
+    /// The theme dark mode setting.
+    pub is_dark: bool,
+    /// The theme auto-switch dark and light mode setting.
+    pub auto_switch: bool,
+}
+
+impl Default for ThemeMode {
+    fn default() -> Self {
+        Self {
+            is_dark: true,
+            auto_switch: false,
+        }
+    }
+}
+
+impl ThemeMode {
+    /// Check if the theme is currently using dark mode
+    pub fn is_dark(config: &Config) -> Result<bool, cosmic_config::Error> {
+        config.get::<bool>("is_dark")
+    }
+
+    /// version of the theme
+    pub fn version() -> u64 {
+        1
+    }
+
+    /// Set auto-switch from light to dark mode
+    pub fn set_auto_switch(config: &Config, value: bool) -> Result<(), cosmic_config::Error> {
+        config.set("auto_switch", value)
+    }
+
+    /// Get the config for the theme mode
+    pub fn config() -> Result<Config, cosmic_config::Error> {
+        Config::new(THEME_MODE_ID, Self::version())
+    }
+}