DEV: Introduce `Theme#get_setting` (#24032)

Why this change?

Currently, we do not have a method to easily retrieve a theme setting's
value on the server side. Such a method can be useful in the test
environment where we need to retrieve the theme's setting and use its
value in assertions.

What does this change do?

This change introduces the `Theme#get_setting` instance method.

app/models/theme.rb

@@ -699,6 +699,26 @@ class Theme < ActiveRecord::Base
     hash
   end
 
+  # Retrieves a theme setting
+  #
+  # @param setting_name [String, Symbol] The name of the setting to retrieve.
+  #
+  # @return [Object] The value of the setting that matches the provided name.
+  #
+  # @raise [Discourse::NotFound] If no setting is found with the provided name.
+  #
+  # @example
+  #   theme.get_setting("some_boolean") => True
+  #   theme.get_setting("some_string") => "hello"
+  #   theme.get_setting(:some_boolean) => True
+  #   theme.get_setting(:some_string) => "hello"
+  #
+  def get_setting(setting_name)
+    target_setting = settings.find { |setting| setting.name == setting_name.to_sym }
+    raise Discourse::NotFound unless target_setting
+    target_setting.value
+  end
+
   def update_setting(setting_name, new_value)
     target_setting = settings.find { |setting| setting.name == setting_name }
     raise Discourse::NotFound unless target_setting

spec/models/theme_spec.rb

@@ -1040,6 +1040,37 @@ HTML
     end
   end
 
+  describe "get_setting" do
+    before do
+      theme.set_field(target: :settings, name: "yaml", value: <<~YAML)
+        enabled:
+          type: bool
+          default: false
+        some_value:
+          type: string
+          default: "hello"
+      YAML
+
+      ThemeSetting.create!(
+        theme: theme,
+        data_type: ThemeSetting.types[:bool],
+        name: "super_feature_enabled",
+      )
+
+      theme.save!
+    end
+
+    it "returns the value of the setting when given a string represeting the setting name" do
+      expect(theme.get_setting("enabled")).to eq(false)
+      expect(theme.get_setting("some_value")).to eq("hello")
+    end
+
+    it "returns the value of the setting when given a symbol represeting the setting name" do
+      expect(theme.get_setting(:enabled)).to eq(false)
+      expect(theme.get_setting(:some_value)).to eq("hello")
+    end
+  end
+
   describe "#update_setting" do
     it "requests clients to refresh if `refresh: true`" do
       theme.set_field(target: :settings, name: "yaml", value: <<~YAML)