Resolves: tdf#102778 by default use breeze_dark when dark themes are requested

Under gtk3 if gtk-application-prefer-dark-theme is enabled or the requested
icon-theme name ends in [-|_]dark

Change-Id: Ia70ea3adc51ac0ec2b26d08fc960c1c7285c338b

vcl/source/app/IconThemeSelector.cxx

@@ -82,6 +82,10 @@ IconThemeSelector::SelectIconThemeForDesktopEnvironment(
         if (icon_theme_is_in_installed_themes(mPreferredIconTheme, installedThemes)) {
             return mPreferredIconTheme;
         }
+        //if a dark variant is preferred, and we didn't have an exact match, then try our one and only dark theme
+        if (mPreferredIconTheme.endsWith("_dark") && icon_theme_is_in_installed_themes("breeze_dark", installedThemes)) {
+            return "breeze_dark";
+        }
     }
 
     OUString themeForDesktop = GetIconThemeForDesktopEnvironment(desktopEnvironment);

vcl/unx/gtk3/gtk3salnativewidgets-gtk.cxx

@@ -2719,8 +2719,19 @@ void GtkSalGraphics::updateSettings( AllSettings& rSettings )
 
     // preferred icon style
     gchar* pIconThemeName = nullptr;
-    g_object_get( pSettings, "gtk-icon-theme-name", &pIconThemeName, nullptr );
-    aStyleSet.SetPreferredIconTheme( OUString::createFromAscii( pIconThemeName ) );
+    gboolean bDarkIconTheme = false;
+    g_object_get(pSettings, "gtk-icon-theme-name", &pIconThemeName,
+                            "gtk-application-prefer-dark-theme", &bDarkIconTheme,
+                            nullptr );
+    OUString sIconThemeName(OUString::createFromAscii(pIconThemeName));
+    if (sIconThemeName.endsWithIgnoreAsciiCase("-dark", &sIconThemeName) ||
+        sIconThemeName.endsWithIgnoreAsciiCase("_dark", &sIconThemeName))
+    {
+        bDarkIconTheme = true;
+    }
+    if (bDarkIconTheme)
+        sIconThemeName += "_dark";
+    aStyleSet.SetPreferredIconTheme(sIconThemeName);
     g_free( pIconThemeName );
 
     aStyleSet.SetToolbarIconSize( ToolbarIconSize::Large );