The background color of your popup can be changed by short code snippet into your theme. This snippet contains all the information of the change you would like to have.
Step 1
Open your Theme editor and navigate to Settings. Click on Custom CSS and paste the code given to you in Step 2.
You can replace the color code below with any color of your preference.
#smartsize-dialog {
background: #f5f1eb;
}
Step 2
Preview the results and if you are happy, save your changes!
Step 3
Save your changes.