MD Settings: allow changing to GTK+ theme on Linux

chrome/app/settings_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Settings-specific strings (included from generated_resources.grd). -->
 <grit-part>
   <!-- About Page -->
   <if expr="chromeos">
     <message name="IDS_SETTINGS_ABOUT_PAGE_DETAILED_BUILD_INFO" desc="Label describing the section that shows detailed information about the current Chrome build.">
       Detailed build information
     </message>
     <message name="IDS_SETTINGS_ABOUT_PAGE_ARC_VERSION" desc="Label for ARC version">
       ARC Version
@@ skipping 159 matching lines @@
   <!-- Appearance Page -->
   <message name="IDS_SETTINGS_APPEARANCE" desc="Name of the settings page which displays appearance preferences.">
     Appearance
   </message>
   <message name="IDS_SETTINGS_EXAMPLE_DOT_COM" desc="Placeholder text for URL input example.">
     www.example.com
   </message>
   <message name="IDS_SETTINGS_SET_WALLPAPER" desc="Name of the control which allows the user to set the wallpaper.">
     Wallpaper
   </message>
-  <message name="IDS_SETTINGS_THEME" desc="Name of the control which allows the user to get a theme for the browser.">
-    Theme
+  <message name="IDS_SETTINGS_THEMES" desc="Name of the control which allows the user to get a theme for the browser.">
+    Themes
   </message>
-  <message name="IDS_SETTINGS_RESET_TO_DEFAULT_THEME" desc="Name of the control which resets the browser theme back to the default theme.">
-    Reset to default
-  </message>
+  <if expr="is_linux and not chromeos">
+    <message name="IDS_SETTINGS_SYSTEM_THEME" desc="Text of the label describing the system (GTK+) browser theme on Linux">
+      GTK+
+    </message>
+    <message name="IDS_SETTINGS_USE_SYSTEM_THEME" desc="Text of the button that switches the browser theme to the system (GTK+) theme on Linux">
+      Use GTK+
+    </message>
+    <message name="IDS_SETTINGS_CLASSIC_THEME" desc="Text of the label describing the classic browser theme on Linux">
+      Classic
+    </message>
+    <message name="IDS_SETTINGS_USE_CLASSIC_THEME" desc="Text of the button that switches the browser theme to the classic theme on Linux">
+      Use Classic
+    </message>
+  </if>
+  <if expr="not is_linux or chromeos">
+    <message name="IDS_SETTINGS_RESET_TO_DEFAULT_THEME" desc="Name of the control which resets the browser theme back to the default theme.">
+      Reset to default
+    </message>
+  </if>
   <message name="IDS_SETTINGS_SHOW_HOME_BUTTON" desc="Label for the checkbox which enables or disables showing the home button in the toolbar.">
     Show home button
   </message>
   <message name="IDS_SETTINGS_SHOW_BOOKMARKS_BAR" desc="Label for the checkbox which enables or disables showing the bookmarks bar in the toolbar.">
     Always show the bookmarks bar
   </message>
   <message name="IDS_SETTINGS_HOME_PAGE_NTP" desc="Description of the New Tab Page when set as the home page.">
     Use the New Tab Page
   </message>
   <message name="IDS_SETTINGS_OTHER" desc="Label of the input box for the home page.">
@@ skipping 2271 matching lines @@
       System
     </message>
     <message name="IDS_SETTINGS_SYSTEM_HARDWARE_ACCELERATION_LABEL" desc="Label for the checkbox that forces Chrome to render via hardware acceleration (GPU) when available.">
       Use hardware acceleration when available
     </message>
     <message name="IDS_SETTINGS_SYSTEM_PROXY_SETTINGS_BUTTON" desc="Text for the button that changes a user's network proxy settings.">
       Change proxy settings
     </message>
   </if>
 </grit-part>