Usual "Dark Reader"-type extensions do not take into account SCP website's intricate and complex structure and custom themes. This one does. This is Dark Reader For RuFoundation - D R F R F.
Warning! Works only for RuFoundation-based website scpfoundation.net or scp.su.
Install through Mozilla Addons website or by downloading and installing through releases page of this repo. Though the last one is not recommended since cannot be updated automatically.