Use output colors that are easy to read on both light and dark backgrounds

[homebysix]

This screenshot reveals that the [WARNING] text is rather hard to read on a white background. I think we may want to review how each output color looks on white, on black, and on medium gray.

[jgstew]

I was definitely noticing this and meant to open an issue about it.

Can you detect the terminal background color at runtime? That might be a useful way to know which colors to use and which to avoid.

It would be interesting if there was a python library for handling text colorization based upon terminal background color automatically.

[homebysix]

Here's a test I put together with slightly different colors. The main difference is the WARNING text is more orange, less yellow.

The blue reminder text doesn't look perfect on the middle gray, but overall it looks good.