On the Ryujin page of AC, above and to the right of the dropdown box where you choose "Hardware Monitor / Image or Animation / Custom", there is a settings button (Gear icon). Click that and you'll get a dialog where you can change the display brightness. Mine is set to 30% and is bright enough to read through the smoked-glass window of my case.
If that doesn't help, try wiggling or removing/replacing the display from the pump. It attaches magnetically, and there are electrical contacts for the display that might not be making good contact.