How to add Custom CSS to WordPress admin dashboard
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https://webcusp.com/how-to-add-custom-css-for-your-wordpress-admin-dashboard/
Way 1 is more comprehensive, and it doesn’t let you stuck anywhere while editing CSS (but requires access to FTP or hosting panel). On the other hand, way 2 is easy, but you may get stuck due to applying wrong CSS. For instance, if you accidentally hide admin wrapper, all content will be disappeared in admin panel. You can’t then edit admin css via admin panel. In that condition, you need to access css file via FTP or hosting file manager. (specially, editing php files via FTP (or file manager) is recommended)