fix: merge cssVar

Author: SSShooter

package.json

@@ -1,6 +1,6 @@
 {
   "name": "mind-elixir",
-  "version": "4.2.4",
+  "version": "4.2.5",
   "type": "module",
   "description": "Mind elixir is a free open source mind map core.",
   "keywords": [

src/utils/theme.ts

@@ -1,7 +1,9 @@
+import { THEME } from '../const'
 import type { MindElixirInstance } from '../types/index'
 import type { Theme } from '../types/index'
 
 const changeTheme = function (this: MindElixirInstance, theme: Theme, shouldRefresh = true) {
+  theme.cssVar = { ...THEME.cssVar, ...theme.cssVar }
   this.theme = theme
   const cssVar = this.theme.cssVar
   const keys = Object.keys(cssVar)