You should be able to do this with Additional CSS under the Customizer, but without seeing your site it is almost impossible to help.
Please note that if you post a link to your site (other than in the designated “The Page I need help with” field in the original post), then that link will be publicly available and will remain so for the foreseeable future, as the current policy is to not edit posts, except for under truly exceptional circumstances.
You just posted 5 minutes ago, so you should still be able to edit your original post for now.
what theme?
have you asked the developer of the theme for help, or posted in that theme’s dedicated support forum?