Here's Signal:
https://imagebin.ca/v/4aAU1ai9CG8D
And Telegram:
https://imagebin.ca/v/4aAUJhRmj8Jx
Signal is white (and no night mode on IOS), the bubbles are undecorated rounded rectangles, there's no background and no option to set one, no stickers, no option to set disappearing messages on the fly, no indicator of when the person was last online. It looks plain.
Telegram has a night mode, options to set background colors, chat messages. It doesn't look plain.
As I've said, if you don't care whether something looks plain, then that is absolutely great and entirely up to you. Lots of people don't care about lots of things and there's really nothing wrong with that.
I like my apps to look nice (and I can, within broad strokes, define what "looks nice" is) and I know this isn't a unique position I occupy.
In Settings, there's a moon in the top right which toggles light/dark modes. This is v2.37.1.0 but the release notes list dark mode as appearing in v2.30.2 which is about 4 months ago.