helium/appearance: re-enable gtk/qt buttons (#531)

the agnostic theme will still be the default, but this
will re-enable the use gtk/qt buttons for whoever wants
to use the GTK/QT themes for whatever reason

fixes imputnet/helium-linux#74

patches/helium/settings/fix-appearance-page.patch

@@ -40,47 +40,3 @@
        {"resetToolbarToDefault", IDS_SETTINGS_RESET_TOOLBAR_TO_DEFAULT},
  #if BUILDFLAG(IS_LINUX)
        {"showWindowDecorations", IDS_SHOW_WINDOW_DECORATIONS},
---- a/chrome/browser/resources/settings/appearance_page/appearance_page.html
-+++ b/chrome/browser/resources/settings/appearance_page/appearance_page.html
-@@ -46,7 +46,6 @@
-               hidden="[[!pageVisibility.setTheme]]"
-               label="$i18n{themes}" sub-label="[[themeSublabel_]]"
-               on-click="onThemeClick_" external></cr-link-row>
--<if expr="not is_linux">
-           <template is="dom-if" if="[[prefs.extensions.theme.id.value]]">
-             <div class="separator"></div>
-             <cr-button id="useDefault" on-click="onUseDefaultClick_"
-@@ -54,33 +53,6 @@
-               $i18n{resetToDefault}
-             </cr-button>
-           </template>
--</if>
--<if expr="is_linux">
--          <div class="settings-row continuation"
--              hidden="[[!showThemesSecondary_(
--                  prefs.extensions.theme.id.value, systemTheme_)]]"
--              id="themesSecondaryActions">
--            <div class="separator"></div>
--            <template is="dom-if" if="[[showUseClassic_(
--                prefs.extensions.theme.id.value, systemTheme_)]]" restamp>
--              <cr-button id="useDefault" on-click="onUseDefaultClick_">
--                $i18n{useClassicTheme}
--              </cr-button>
--            </template>
--            <template is="dom-if" if="[[showUseGtk_(
--                prefs.extensions.theme.id.value, systemTheme_)]]" restamp>
--              <cr-button id="useGtk" on-click="onUseGtkClick_">
--                $i18n{useGtkTheme}
--              </cr-button>
--            </template>
--            <template is="dom-if" if="[[showUseQt_(
--                prefs.extensions.theme.id.value, systemTheme_)]]" restamp>
--              <cr-button id="useQt" on-click="onUseQtClick_">
--                $i18n{useQtTheme}
--              </cr-button>
--            </template>
--          </div>
--</if>
-         </div>
-         <div id="toolbarRow" class="settings-row">
-           <cr-link-row id="customizeToolbar"