This is the default behavior of the theme you are using. You can select another theme from the Look & Feel tab in the survey builder in the upper left corner. Here is a link to the documentation for more detail:

https://qualtrics.com/support/survey-platform/survey-module/look-feel/applying-survey-themes/

You can also have a custom theme created if it is part of your license. Documentation here:

https://qualtrics.com/theme-design/

If you are interested in how to do this with custom css code you can post your questions to the Custom Code section in the Developer Corner here in the community.

---