This issue developed just recently using the latest version of Opera: LVL 4 (core: 89.0.4447.64). Changing from light to dark themes has no effect.
OS: Win 11 Home, V 21H2, OS Build 22000.795
Processor: AMD Ryzen 9 5900 12-Core Processor 3.00 GHz
RAM 32.0 GB
Graphics Card: Nvidia GeForce RTX 3090