A theme is wallpaper and icons, so you must have installed something that installed a theme. You can't remove a theme, you mus have wallpaper, even if it's just solid black, and you must have icons. If you don't like the theme you have, you have to replace it.
But your problem is probably the app that installed the theme. Think back to what you installed recently and uninstall any app that may have been the one you installed around the same time.
For themes, you can use the Wallpaper section of your launcher (just long-press a blank spot and choose Wallpapers). For icons, look in Play - search
icons. (If your launcher can't take icons (many can't), either stick with the ones you have or, if you find a set you'd like better, install a different launcher., one that can be themed (Many of us use
Nova Launcher.)