Hello @michelles94 – thanks for writing in to us!
You can change the text color or the background color of the popup by browsing Popup Maker > Popup Themes.
From there, select the theme you want to customize and click Edit.
(a) Click Container tab > Background to change the background color of the popup.
(b) Click Title or Content tab to change the text color.
See this demo screencast from my test site: https://share.getcloudapp.com/v1uLn0r8
You can try that and hope that helps you with your query.
If you have further questions, don’t hesitate to let us know.
Cheers