In https://sprintr.home.mendix.com/link/myapps you have to create the application. After opening the application go to edit in studio you can find a symbol below the search . I had sent the image for reference. Hope it helps!!!!
Lanavya’s answer is until version 9.20. For 9.21 you should look in Studio Pro under Styles. I opened the place to edit the progressive web style.
See learning path https://academy.mendix.com/link/paths/123/Style-your-App-with-Sass and https://academy.mendix.com/link/paths/122/Polish-your-Styling for learning paths handling the styling of Apps.
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