provide the option to apply RON theme files via the cli

just run 'cosmic-settings --import-theme /path/to/theme.ron' to apply a
theme from the cli without launching the COSMIC settings app GUI. This
is especially helpful for anyone using dotfiles.

cosmic-settings/src/main.rs

@@ -21,6 +21,8 @@ pub mod theme;
 pub mod utils;
 pub mod widget;
 
+use std::path::PathBuf;
+
 use clap::{Parser, Subcommand};
 use cosmic::{app::CosmicFlags, iced::Limits};
 use i18n_embed::DesktopLanguageRequester;
@@ -34,6 +36,11 @@ use tracing_subscriber::prelude::*;
 pub struct Args {
     #[command(subcommand)]
     sub_command: Option<PageCommands>,
+
+    /// Import a theme from a RON file and exit without starting the GUI
+    #[arg(long, value_name = "FILE")]
+    #[serde(skip)]
+    import_theme: Option<PathBuf>,
 }
 
 #[derive(Subcommand, Debug, Serialize, Deserialize, Clone)]
@@ -175,6 +182,10 @@ pub fn main() -> color_eyre::Result<()> {
 
     let args = Args::parse();
 
+    if let Some(theme_path) = &args.import_theme {
+        return import_theme_cli(theme_path);
+    }
+
     let settings = cosmic::app::Settings::default()
         .size_limits(Limits::NONE.min_width(360.0).min_height(300.0));
 
@@ -189,6 +200,80 @@ pub fn main() -> color_eyre::Result<()> {
     Ok(())
 }
 
+fn import_theme_cli(path: &std::path::Path) -> color_eyre::Result<()> {
+    use cosmic::cosmic_config::CosmicConfigEntry;
+    use cosmic::cosmic_theme::{Theme, ThemeBuilder, ThemeMode};
+
+    if !path.exists() {
+        return Err(color_eyre::eyre::eyre!(
+            "Theme file not found: {}",
+            path.display()
+        ));
+    }
+
+    if path.extension().is_none_or(|ext| ext != "ron") {
+        return Err(color_eyre::eyre::eyre!(
+            "Theme file must have .ron extension: {}",
+            path.display()
+        ));
+    }
+
+    let content = std::fs::read_to_string(path).map_err(|e| {
+        color_eyre::eyre::eyre!("Failed to read theme file '{}': {}", path.display(), e)
+    })?;
+
+    let builder: ThemeBuilder = ron::de::from_str(&content).map_err(|e| {
+        color_eyre::eyre::eyre!("Failed to parse theme file '{}': {}", path.display(), e)
+    })?;
+
+    let is_dark = builder.palette.is_dark();
+    let mode_str = if is_dark { "dark" } else { "light" };
+
+    if let Ok(mode_config) = ThemeMode::config() {
+        let mut mode = ThemeMode::get_entry(&mode_config).unwrap_or_default();
+        if mode.is_dark != is_dark {
+            let _ = mode.set_is_dark(&mode_config, is_dark);
+        }
+    }
+
+    let (theme_config, builder_config) = if is_dark {
+        (Theme::dark_config(), ThemeBuilder::dark_config())
+    } else {
+        (Theme::light_config(), ThemeBuilder::light_config())
+    };
+
+    if let Ok(config) = builder_config {
+        builder.write_entry(&config).map_err(|e| {
+            color_eyre::eyre::eyre!("Failed to write theme builder config: {:?}", e)
+        })?;
+    } else {
+        return Err(color_eyre::eyre::eyre!(
+            "Failed to get {} theme builder config",
+            mode_str
+        ));
+    }
+
+    let theme = builder.build();
+    if let Ok(config) = theme_config {
+        theme
+            .write_entry(&config)
+            .map_err(|e| color_eyre::eyre::eyre!("Failed to write theme config: {:?}", e))?;
+    } else {
+        return Err(color_eyre::eyre::eyre!(
+            "Failed to get {} theme config",
+            mode_str
+        ));
+    }
+
+    println!(
+        "Successfully imported {} theme from: {}",
+        mode_str,
+        path.display()
+    );
+
+    Ok(())
+}
+
 fn init_localizer() {
     let localizer = crate::localize::localizer();
     let requested_languages = DesktopLanguageRequester::requested_languages();