Improve the way of using color schemes.

- Gracefully switch code color scheme in the future.
- Also uniform border color.

assets/css/_variables.scss

@@ -7,6 +7,8 @@
 * MIT Licensed
 */
 
+/*--- ↓ width and height ----*/
+
 $sidebar-width-small: 210px;
 $sidebar-width-medium: 260px;
 $sidebar-width-large: 350px;
@@ -19,4 +21,4 @@ $main-content-max-width: 1150px;
 
 $panel-max-width: 300px;
 
-$post-extend-min-height: 35rem;
+$post-extend-min-height: 35rem;