How to change the tag banner style is explained here: Discourse Tag Banners
You can change the background from the CSS editor on your site. Go to admin/customize/themes/1/common/scss/edit and add your CSS there:
For example, I used:
[class*="tag-title-header"][class*=" tag-banner-"] {
background-color: var(--secondary);
}
With attribute selectors, so the background is changed in all tags. But you could specify a tag name in the class so the background is changed for only that tag.