No it’s not the same, have you ever used a wheelchair or known a user? Many many adaptions have to be made to basically every building for that wheelchair to be functional. Curb cuts on sidewalks, buttons on doors, elevators everywhere. Simply “getting into a wheelchair” isn’t enough to make accessibility work, you need to fundamentally change the infrastructure.

Don’t put the disabled in the position where the website itself takes extra steps just to be usable for them, old 90 year old grandma Betty with failing vision is never going to figure such things out and saying “lol just figure it out” isn’t good enough. Those extensions that repaint websites with a new color palette are ugly as crap and buggy, and anybody with the attitude their website doesn’t need to follow WCAG by default definitely hasn’t actually checked such extensions actually work. The burden of trying to make sure every such extension is compatible is dramatically harder than just making the site accessible to begin with, the entire approach is bad design, it’s harder for everybody. What you’re doing is less building a wheelchair ramp and more leaving a pile of lumber next to some stairs, building the wheelchair ramp would mean having an accessible theme be the default and maybe the OPs theme be an alternate.

The analogy I would make is actually round doorknobs vs door levers/handles. If you pay attention you will notice you don’t see doorknobs on new public buildings and that’s because of issues like arthritis. Yes somebody might aesthetically prefer doorknobs but that doesn’t justify their installation in something used by the general public. It’s fine for a PRIVATE website to be as inaccessible as you want, but not a public one.