change color of download box on checkout order received page

Please could you post the url to your site and put up a free product, that you won’t supply, to enable access to the thank you page.

I dont have a store on my site but here is a link to a free test product. If you follow through checkout the last page has the download button that Im referring to.
https://www.glencumbus.com/product/test-product/

Try this custom css:

#main-wrapper .hb-main-content a.woocommerce-MyAccount-downloads-file {
  color: White !important;
  background-color: rgb(46, 204, 113);
}

Custom css can be entered at:
Dashboard > Appearance > Customise > Additional CSS

Hey, that worked. Can I change the background color to something else by changing the numbers in there? Thanks.

Sure.