RokuDanny-R, I have to be honest, this isn't really a solution. this post was about the build-in Roku app subs, not the individual channel that uses them.
the basic subtitles provided by Roku are, to be quite frank, lacking. their 'white' option is actually more of a gray, they lack a sufficiently bold font, and the outline is simply too thin. I myself was also watching something on Funimation last night, and missed several lines because of how unreadable the text can be at times. And I must echo the sentiment of the previous poster, adding a background simply obscures more of the picture, which is less than desirable.
As a fairly new Roku customer, I've been pleased with almost every other aspect of the device so far (and the few I haven't been pleased with are either due to bad internet connection, which is a problem on my end, or a problem specific to the apps, which is on their end), but the default subtitles that come with the Roku are simply lacking.
One last complaint I suppose I have is that they can't be toggled per app, and must be always on or always off. This is annoying, as I really only need them when watching something in a language other than English, but it's a minor complaint and I can work around it. the subtitle font problem, though, I fear renders some content simply unwatchable.