Developer tools allow you to add custom CSS and JS code to your site. The tools are not available to you by default, so here are the steps to gain access:
Note: Only Owner and Admin roles can access the Settings panel. If you are a Contributor, you will need to ask the site Admin to grant you Developer permissions.
1. Select Settings (gear icon) in the Dashboard navigation bar.
2. Select Users & Roles under the Settings header.
3. Select the edit link next to the user email you want to grant access to: