I rebuilt Bitcointalk's design with my own code. I used CSS gradients instead of images (that Bitcointalk uses right now). I used a combination of Flexbox and CSS grid (mostly flexbox) instead of very old-fashioned HTML tables. Added a small button and with the help of some js, I implemented Dark and Light mode toggler.
I think the SMF software bitcointalk is using has been customized with so many modules that implementing a new theme will loss a lot of features we have now. It will require collecting all the features, documenting and backup the codes of the features those were added then think about implementing a new theme. It's good to see many are trying their best.