refactor(core): adjust directory structure:

src/core/util/polyfill/css-vars.js

@@ -0,0 +1,30 @@
+import { get } from '../fetch/ajax'
+
+function replaceVar (block) {
+  block.innerHTML = block.innerHTML.replace(/var\(\s*--theme-color.*?\)/g, $docsify.themeColor)
+}
+
+export function cssVars () {
+  // variable support
+  if (window.CSS && window.CSS.supports && window.CSS.supports('(--foo: red)')) return
+
+  const styleBlocks = document.querySelectorAll('style:not(.inserted),link')
+
+  ;[].forEach.call(styleBlocks, block => {
+    if (block.nodeName === 'STYLE') {
+      replaceVar(block)
+    } else if (block.nodeName === 'LINK') {
+      const href = block.getAttribute('href')
+
+      if (!/\.css$/.test(href)) return
+
+      get(href).then(res => {
+        const style = document.createElement('style')
+
+        style.innerHTML = res
+        document.head.appendChild(style)
+        replaceVar(style)
+      })
+    }
+  })
+}