Hi there!
Thanks for your query.
You have a solid color in the header already here: https://www.tommycandtheblackwidowgreaseband.com/
Though it's Indigo themplate there, not Rhythm as I can see.
Can you give me the site URL where you want to change.
However, if you are using predefined header then you can use the following code in your custom CSS:
#sp-header {
background: red !important;
}
Please use your desired color in the above code.
Note: Path towards custom CSS(Joomla 3): Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.
Best Regards