@Jeyycie : Hello Jeyycie! I see now. What you are referring to is your Overlay Menu. You should be able to change the colors without needing CSS. Here are the steps...
1. Enter 'Edit' mode
2. Select 'Edit Site Header'
3. A menu should appear. Select the mobile icon on the top right
4. Select 'Overlay Menu'
5. Select 'Colors'
6. Choose your color palette. If needed, you can edit our color palette in Site Styles
