This is trivial, but extremely easy to fix.
Presentation -> Theme Editor page, the following text displays if the files in question don't have the correct priviledges set:
If this file was writable you could edit it.
The correct english would be:
If this file were writable you could edit it.
As I said, trivial, but it grates on my ears...
(for those who care, that's an "if clause contrary to fact," and thus requires the imperfect subjunctive rather than the past tense of the verb