The Chess.com wallpapers are at https://addons.opera.com/en/wallpapers/details/chess-dark/ and https://addons.opera.com/en/wallpapers/details/chess-light/. You can install the one you want in Opera.

If you don't have the Chess.com speed dial, you can click the + button on the start page to create a new speed dial for the URL https://chess.com/. Or, if you want to give Opera credit, set the URL to https://redirector.opera.com/redirect?type=syn&rid=719 or https://www.chess.com/?utm_source=partnership&utm_medium=opera&utm_campaign=speed_dial instead. Then, you can right-click the dial, choose "Edit" and name it Chess.com. Then, you can right-click the speed dial, choose "Set custom image" and point it to https://sd-images.operacdn.com/api/v1/images/a17380dbb9b066104ce4eb1d65b601f100025a12.png (that you download first). You can then drag the speed dial to the first spot on the start page if you want.

If you want chess.com in the sidebar, you can goto the URL opera://flags/#sidebar-site-panel, enable the flag, restart Opera, click the 3 dots at the bottom of the sidebar, scroll down to "Custom Site Panels" and add the URL https://www.chess.com/.