fix(Core): 修复使用 css variable 模式构建时，主题包内的 varMap 配置值会被重复覆盖的问题 (#5002)

Author: YSMJ1994

components/core/util/_varMap.scss

@@ -1,4 +1,4 @@
-$varMap: ();
+$varMap: () !default;
 @function get-compiling-value($cssVarRepresentation) {
     @if map-has-key($varMap, $cssVarRepresentation) {
         @return map-get($varMap, $cssVarRepresentation);