You may have to rebuild the stylesheet definitions of your theme after you have upgraded your ABAP system to a higher support package (SP) or implemented a patch for the rendering of one of the supported UI technologies.
Context
Your themes are always based on SAP themes. If the SAP theme on which your theme is based was upgraded to a new version with a higher SP stack or patch, rendering issues may occur. To avoid them, you need to rebuild the CSS files of your theme.