based on your feedback I have tried different versions of Opera in a VirtualBox. The two wallpapers "style snowflake" and "Wallpaper with a Pattern" can be installed up to version 62.0.3331.99 of July 24, 2019, from the first 63er version – i.e. version 63.0.3368.35 of August 20, 2019 – this is no longer possible.
I have now installed Opera 62.0.3331.99 and "switched off" the update function to be able to install the (in my eyes very nice) wallpaper "Wallpaper with a Pattern".
I have only recently started using Opera as a second browser. What I don’t understand is that there are only two different themes – "light" and "dark". This is very spartan and really unsatisfactory. Even though there are many wallpapers for Opera, they are not themes by no means. With Chrome you can use countless themes, even with Edge you can use themes from Chrome. I see an urgent need for improvement in the treatment of themes in Opera!