I saw this answer too late, so I prepared php procedure to find and decrease brightness of every color from original css.
opus_darker.zip (270.9 KB)
Use Firefox Stylish to add new style and paste opus80_stylish.txt from inside archive.
I prepared also other brightness (from 70% to 90%). Just replace any of them inside that style in txt except first and last line (they're important). Someone can change it to work with Chrome too (if someone wants).
Effect looks like that: