chore: adjust sidebar's fontSize (#306)

Author: mortalYoung

.dumi/theme/style/const.scss

@@ -1,4 +1,4 @@
 $primary-color: #1d78ff;
 $basic-black-color: #3d446e;
 $navbar-font-size: 16px;
-$sidebar-font-size: 16px;
+$sidebar-font-size: 14px;