huuduy216 You could in theory do that, a better option however would be to create an extension, with a layout like:
Custom CSS Extension
|-- css
| `-- custom.css
`-- extend.php
For the directory structure. Then in the extend.php file you would need:
<?php
use Flarum\Extend;
return [
(new Extend\Frontend('forum'))
->css(__DIR__.'/css/custom.css')
];