You can change the mode with the button that says something like "Mode: auto" or by pressing F10 once or twice.
Making the tileset change color is much trickier, but technically possible with advanced scripting techniques - if you make multiple versions of the same room with different colors, there's the internal scripting command `gotoroom(x,y)` which you could run on some kind of timer. But it requires quite some explanation and layers of background knowledge; how much do you know about (internal) scripting?
I did read your note; the rainbow room I see in Cloning Technology is really the Lab background.
To enable it, in a room with the Lab tileset, set the mode to either manual or multi (not auto), then cycle through the colors until it says Rainbow BG.
The tower tileset is not normally selectable in custom levels, but you can customize the graphics assets for your level to make other tilesets look like the tower.
Which level did you see the rainbow background in?
Yeah, dropping XP wasn't a "decision" that was made, usually OS versions get dropped by factors outside of your control, or as side-effects of compiler updates... Even for old games.
But you're in luck, because nothing had actually changed about how the game was built in 2.4 for Windows, and it turned out to be pretty simple to fix. Wouldn't be surprised if it becomes unfeasible to keep supporting it in the future, though.
A bit of a clarification on this issue: Javascript internally uses UTF-16 for strings, so characters until U+FFFF work as expected. Above that though, they're encoded in pairs of characters from U+D800 to U+DFFF, which are defined to be invalid codepoints otherwise. So characters from U+10000 onward will end up in the TTF as U+Dxxx, and the rest of the characters on that line will be misaligned.
