A new SteamOS update was released to allow us to adjust the brightness of the charging LED on the Steam Deck OLED.
New update on the SteamOS beta channel, lets OLED users control the brightness of the charging led. Also includes a fix for WiFi failing to connect and an issue with HDR through some display port connections becoming desaturated.
The OLED has an upgraded led that can do different colors and brightnesses. I don't know if the mono-color LCD led can change brightness, but I imagine it can't.