Card visual loses formatting, background and foreground, when pinned to a dashboard.
Pin visual from a report to a dashboard will lose visual background color. To work around the issue, we can use Pin Live Page feature.
Also you can vote this similar idea: https://ideas.powerbi.com/forums/265200-power-bi-ideas/suggestions/6450540-change-background-color-o...
Best Regards,Qiuyun Yu