If you really want to continue with subsilver you could try Allan Style (viewtopic.php?f=581&t=2412546) which inherits from prosilver. I haven't tried it so no guarantees.
BTW the last version is for 3.2.1 (and as with any custom style it will take some time before it is updated to the latest version of phpBB).
Spelling is freeware, which means you can use it for free.
On the other hand, it is not open source, which means you cannot change it or publish it in a modified form.
Time flies like an arrow, but fruit flies like a banana.
In addition I have found Black Letters on a Light Yellow Background beneficial to people with Macular Degeneration. (light blue has also been cited as helpful)