My vision is getting poor and I appreciate the outline I used to get on a selected channel. Now I have to either guess which of 3 are highlighted or read the text. It's not to the terrible point now, mostly because I have a large screen tv, but it was easier & I dunno why it just went "poof"
There have been a few reports of this. Usually restarting your Roku from Settings->System or Settings->System->Power (as appropriate) clears it up.
Was your Roku wallpaper recently changed?