Some sites are beautifully, extensively themed, like Blizzards or say, https://community.festingervault.com
I do wonder however, if once you add a logo and appropriate base colours to match branding, a redesign of the rest of the interface is lower priority because the thing most people care about is a nice, tidy and functional UI that works on touch devices and is clear to read.
Yes, you can spend $2k to $10k on intricate bespoke theming, but do you generally need to?
The base Light or Dark theme do 95% of what you need? The more important element is the actual content that is written?