@expvt GX Cleaner shows up in the sidebar if enabled, never used it myself. You can click the icon that looks like 3 bars at the end of the address bar to change themes, or also right-ckick on the speed dial background and select Change wallpaper to get to the same panel (Easy setup). Alternatively you can right-click on an image and choose "Use image as wallpaper" from the menu. The Opera add-ons site also includes some animated themes; if you select "Get more wallpapers" it'll show them to you.
Uninstall Opera (keep your data if you want). Then, download the offline installer from https://www.opera.com/download, launch the installer, click "options", set things how you like and install. If that doesn't work, you can rename your Opera profile folder if you want so Opera creates a new one. If that doesn't help, you can put the old one back.
@daxagoro Possibly the site itself tries to resize things for your window?
This computer is 1366x768; fairly typical resolution for a laptop. My actual window is probably 1150-1200 wide (never actually measured it) - if your window was less than about 1000 pixels wide they might start resizing stuff to fit.