Add dark theme switch in preferences

gajim/data/gui/preferences_window.ui

@@ -1508,9 +1508,6 @@ $T will be replaced by auto-not-available timeout</property>
         <property name="margin_bottom">18</property>
         <property name="orientation">vertical</property>
         <property name="spacing">12</property>
-        <child>
-          <placeholder/>
-        </child>
         <child>
           <object class="GtkFrame" id="frame3">
             <property name="visible">True</property>
@@ -1616,6 +1613,28 @@ $T will be replaced by auto-not-available timeout</property>
                     <property name="top_attach">0</property>
                   </packing>
                 </child>
+                <child>
+                  <object class="GtkCheckButton" id="enable_dark_theme">
+                    <property name="label" translatable="yes">Enable dark theme</property>
+                    <property name="visible">True</property>
+                    <property name="can_focus">True</property>
+                    <property name="receives_default">False</property>
+                    <property name="halign">start</property>
+                    <property name="use_underline">True</property>
+                    <property name="draw_indicator">True</property>
+                    <signal name="toggled" handler="on_enable_dark_theme_toggled" swapped="no"/>
+                  </object>
+                  <packing>
+                    <property name="left_attach">1</property>
+                    <property name="top_attach">3</property>
+                  </packing>
+                </child>
+                <child>
+                  <placeholder/>
+                </child>
+                <child>
+                  <placeholder/>
+                </child>
                 <child>
                   <placeholder/>
                 </child>
@@ -1640,9 +1659,6 @@ $T will be replaced by auto-not-available timeout</property>
             <property name="position">1</property>
           </packing>
         </child>
-        <child>
-          <placeholder/>
-        </child>
       </object>
       <packing>
         <property name="position">4</property>

gajim/gtk/preferences.py

@@ -24,6 +24,8 @@ from gajim.common import helpers
 from gajim.common import configpaths
 from gajim.common import config as c_config
 from gajim.common import idle
+from gajim.gtk.util import get_dark_theme
+from gajim.gtk.util import set_dark_theme
 from gajim.gtk.util import get_builder
 from gajim.gtk import AspellDictError
 from gajim.gtk.themes import Themes
@@ -207,6 +209,9 @@ class Preferences(Gtk.ApplicationWindow):
         st = app.config.get('use_transports_iconsets')
         self.xml.get_object('transports_iconsets_checkbutton').set_active(st)
 
+        # Dark theme
+        self.xml.get_object('enable_dark_theme').set_active(get_dark_theme())
+
         ### Personal Events tab ###
         # outgoing send chat state notifications
         st = app.config.get('outgoing_chat_state_notifications')
@@ -655,6 +660,9 @@ class Preferences(Gtk.ApplicationWindow):
         self.on_checkbutton_toggled(widget, 'use_transports_iconsets')
         gtkgui_helpers.reload_jabber_state_images()
 
+    def on_enable_dark_theme_toggled(self, widget):
+        set_dark_theme(widget.get_active())
+
     def on_outgoing_chat_states_combobox_changed(self, widget):
         active = widget.get_active()
         old_value = app.config.get('outgoing_chat_state_notifications')

gajim/gtk/util.py

@@ -230,3 +230,13 @@ def convert_rgb_to_hex(rgb_string):
     green = int(rgb.green * 255)
     blue = int(rgb.blue * 255)
     return '#%02x%02x%02x' % (red, green, blue)
+
+
+def set_dark_theme(enable: bool) -> None:
+    settings = Gtk.Settings.get_default()
+    settings.set_property('gtk-application-prefer-dark-theme', enable)
+
+
+def get_dark_theme() -> bool:
+    settings = Gtk.Settings.get_default()
+    return settings.get_property('gtk-application-prefer-dark-theme')