its alot of effort to make them, its not simple, since most forums use a standard, they have the standard additional "themes" implemented from the previously built efforts. so its quick easy and effective.
Since "we" decided to customize our main look to specific rollitup colors if we implement the "themes" where you choose you find.alot of things broken/ unable to read which means we have to comb through the entire website everyday when its taken offline to figure out and recode the themes. which means I have to get the owner to take down the website, recode several new themes than i need to comb through every inch of rollitup and debug it all its sooooo much work like hours and hours
in essence its an extreme amount of work and it will be unlikey to implemented since it doesnt effect much but a few people who want a darkmode.
I believe a few people figure out how to put it into dark mode on a specific browser though