I can reproduce this issue in my viva connection. By design, after changing the theme color in the SharePoint home page, you should click "refresh" button in the viva connection to display newly changed theme color.
You could submit a feedback in this portal to improve product.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.