Basically, I need an option for my users to disable the sidebar, while looking at the posts. I can do that now, but that would disable it for everyone, I'd like to offer that feature to only those who need it.

Because some people, with low 4:3 resolution, will see my images (that are 800px wide) in the sidebar.

I tried installing two Atahualpa themes and a theme switcher. Then I edited one theme's settings that it would not show a sidebar in posts, but when I switched back, I noticed that those settings "carried over," which means it either showed the sidebar for both or for neither of them.

Is there any way I could solve this problem?

Alternatively, could I change the sidebar, so it would be up or down instead? And possibly, so I could only change that in one theme, so people could use that Atahualpa theme, which has the sidebar on the side.

Could you possibly use a gallery with thumbnails and have the images open as a lightbox-style. That way you could have a very narrow sidebar containing a 1 x any-number gallery and could accommodate users with low res screens.

I'm afraid not, full-sized images are kind-of needed for my blog, which is used for my "let's play."

Since there are screenshots from the visual novel from time to time, I feel like it would stop the flow, if people had to click to open the images or anything of that sorts.

It also has youtube videos. While I can make those smaller, some people don't like to use full-screen, so I'd rather use the proper resolution for that.

Thinking outside the box....

Create two style sheets, not two themes with switching. Similar to a Mobile option I guess.

How would I be able to create links for switching them?

And from what I saw from the css files, I didn't see the part where sidebars where handled.

That option, would be the best, to be honest, if it works on the fly (switching with AJAX?).

Another way might be to create two "all-encompassing" classes and then replicate the CSS generation with different page widths/sidebar generation for each class.

Its a problem I have to overcome at a later date as I need to add mobile-css to my own site. Thankfully I do not have to deal with this task myself yet!

I was just thinking out loud and in general terms because I will have to think about it myself at a later date.

Good luck.