@Jessica speake Welcome to the Atlassian community.
Although this question is not related with Confluence Cloud, but with Confluence server, I can help you.
I don't think there is any feature to change color of table header. But if you want to change it for all available tables you can use this link - https://confluence.atlassian.com/confkb/how-to-change-table-appearance-using-css-313464913.html
On side note, there are many add-on (apps) are available on marketplace to help you create better looking tables.
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