Fix transparent editor theme being brighter

(cherry picked from commit 0222f7848219616550747f2adb8cf585f2a009fd)

editor/editor_themes.cpp

@@ -559,7 +559,9 @@ Ref<Theme> create_editor_theme(const Ref<Theme> p_theme) {
 	style_tab_disabled->set_border_color(color_disabled);
 
 	// Editor background
-	theme->set_stylebox("Background", "EditorStyles", make_flat_stylebox(background_color, default_margin_size, default_margin_size, default_margin_size, default_margin_size));
+	Color background_color_opaque = background_color;
+	background_color_opaque.a = 1.0;
+	theme->set_stylebox("Background", "EditorStyles", make_flat_stylebox(background_color_opaque, default_margin_size, default_margin_size, default_margin_size, default_margin_size));
 
 	// Focus
 	Ref<StyleBoxFlat> style_focus = style_default->duplicate();