The most simple way IMHO is to use black font on light(-ish) or white font on dark(-ish) background. More universal font (for best contrast) is white with blac

The most simple way IMHO is to use black font on light(-ish) or white font on dark(-ish) background. More universal font (for best contrast) is white with black outline, though it does not always work either and can look like a meme.