I'm a newbie'ish sorta user to all this but I just thought I'd post here how I solved this "If this file were writable you could edit it" problem.
They key for me was reading the extra info from Yosemite (above).
Changing the permissions for the THEMES folders to 777 or 666 or 755 appeared to do nothing.
Even setting the permissions for the individual theme folders did nothing. Then I read that changing this only effects NEW files... Ah Ha !!!
So in Filezilla, I went into the folder for the theme I was using at my site http://timdotnet.net/wiggumdaily/ (Earthy) and selected all the individual files in there and changed the CHMOD to 666 for all those existing files and BINGO-BANGO, I can now edit the theme from within WP :)
I'll probably stick to doing it using Filezilla, but it's handy to at least be ABLE to quickly make changes from the WP interface.