You didn't answer my 2nd paragraph. I set a channel background to use a dark red background, and set the editbox background to also be red, and I now had a white stripe between them.

Using that option I mentioned, setting it to "no frame" didn't make that go away, but changing "border color" to also be red - caused the white stripe to go away.

As for setting it to a 24-bit color, the dropdown for setting the border color has all the 99 colors in it, so if you're trying to theme something besides plain white or black, that there'd be one of the colors which is what you'd need.