Allow negative contrast values in the editor theme settings

When using a negative contrast value, the base color will be lightened
to create the derivative colors instead of being darkened.

This can lead to better-looking themes, especially for light themes.

(cherry picked from commit e7e2ef07676822d848b8916121033fae559b68b2)

editor/editor_settings.cpp

@@ -385,7 +385,7 @@ void EditorSettings::_load_defaults(Ref<ConfigFile> p_extra_config) {
 	_initial_set("interface/theme/accent_color", Color(0.41, 0.61, 0.91));
 	hints["interface/theme/accent_color"] = PropertyInfo(Variant::COLOR, "interface/theme/accent_color", PROPERTY_HINT_NONE, "", PROPERTY_USAGE_DEFAULT);
 	_initial_set("interface/theme/contrast", 0.25);
-	hints["interface/theme/contrast"] = PropertyInfo(Variant::REAL, "interface/theme/contrast", PROPERTY_HINT_RANGE, "0.01, 1, 0.01");
+	hints["interface/theme/contrast"] = PropertyInfo(Variant::REAL, "interface/theme/contrast", PROPERTY_HINT_RANGE, "-1, 1, 0.01");
 	_initial_set("interface/theme/relationship_line_opacity", 0.1);
 	hints["interface/theme/relationship_line_opacity"] = PropertyInfo(Variant::REAL, "interface/theme/relationship_line_opacity", PROPERTY_HINT_RANGE, "0.00, 1, 0.01");
 	_initial_set("interface/theme/highlight_tabs", false);