Make main editor window border margin controllable by theme

editor/editor_node.cpp

@@ -736,6 +736,8 @@ void EditorNode::_notification(int p_what) {
 				get_window()->set_theme(theme);
 
 				gui_base->add_theme_style_override("panel", gui_base->get_theme_stylebox(SNAME("Background"), SNAME("EditorStyles")));
+				main_vbox->set_anchors_and_offsets_preset(Control::PRESET_FULL_RECT, Control::PRESET_MODE_MINSIZE, gui_base->get_theme_constant(SNAME("window_border_margin"), SNAME("Editor")));
+				main_vbox->add_theme_constant_override("separation", gui_base->get_theme_constant(SNAME("top_bar_separation"), SNAME("Editor")));
 				scene_root_parent->add_theme_style_override("panel", gui_base->get_theme_stylebox(SNAME("Content"), SNAME("EditorStyles")));
 				bottom_panel->add_theme_style_override("panel", gui_base->get_theme_stylebox(SNAME("BottomPanel"), SNAME("EditorStyles")));
 				tabbar_panel->add_theme_style_override("panel", gui_base->get_theme_stylebox(SNAME("tabbar_background"), SNAME("TabContainer")));
@@ -7001,8 +7003,8 @@ EditorNode::EditorNode() {
 
 	main_vbox = memnew(VBoxContainer);
 	gui_base->add_child(main_vbox);
-	main_vbox->set_anchors_and_offsets_preset(Control::PRESET_FULL_RECT, Control::PRESET_MODE_MINSIZE, 8);
-	main_vbox->add_theme_constant_override("separation", 8 * EDSCALE);
+	main_vbox->set_anchors_and_offsets_preset(Control::PRESET_FULL_RECT, Control::PRESET_MODE_MINSIZE, gui_base->get_theme_constant(SNAME("window_border_margin"), SNAME("Editor")));
+	main_vbox->add_theme_constant_override("separation", gui_base->get_theme_constant(SNAME("top_bar_separation"), SNAME("Editor")));
 
 	title_bar = memnew(EditorTitleBar);
 	main_vbox->add_child(title_bar);

editor/editor_themes.cpp

@@ -626,6 +626,8 @@ Ref<Theme> create_editor_theme(const Ref<Theme> p_theme) {
 	theme->set_constant("dark_theme", "Editor", dark_theme);
 	theme->set_constant("color_picker_button_height", "Editor", 28 * EDSCALE);
 	theme->set_constant("gizmo_handle_scale", "Editor", gizmo_handle_scale);
+	theme->set_constant("window_border_margin", "Editor", 8);
+	theme->set_constant("top_bar_separation", "Editor", 8 * EDSCALE);
 
 	// Register editor icons.
 	// If the settings are comparable to the old theme, then just copy them over.