add theming documentation

Author: eidng8

README.md

@@ -17,3 +17,28 @@ While working on long list of items, lags will be obvious.
 rendered, sub-trees are controlled by CSS, no DOM structure happens.
 * `v-if` components will lag whenever sub-trees are expanded, every time they
 are expanded.
+
+
+## Theming
+
+This component provides a dark theme out of box. To use it, just add the
+`g8-tree__dark` class to the element.
+
+```html
+<ul class="g8-tree-view g8-tree__dark">
+  <g8-tree-view></g8-tree-view>
+</ul>
+```
+
+If you want to change the color of the component, just defined two variables
+before importing the scss file.
+
+```scss
+/* index.scss */
+
+/* define these two variables before importing the scss file */
+$g8-tree-bg: #ccc !default;
+$g8-tree-fg: #333 !default;
+
+@import "~vue-tree/src/components/tree-view.scss";
+```