is there an optimal font/color scheme for ebooks?

It's not an area I know deeply, but it's my understanding that certain fonts are more generally "legible" than others (for most people). This study found Arial and Microsoft Sans Serif to "be good and legible fonts" and Tahoma to perform poorly. But overall conclude that as long as people are literate, it doesn't make that much of a difference.

On a personal level, I do find certain fonts easier to read (especially for complex, run on sentences in journals) but when reading a novel a thematic font can help set the tone. It can even slow down my temptation to skim read and force me to focus on each word, which can be satisfying. So, it's part preference and part asking what you want the font to do for you - bland, familiar, accessible, easy to skim over? Arial is a good bet. Don't want people to fee like their in the office while reading your novel? Go with something else.

I always mess around with available fonts and try out the publisher font of available, but somehow I almost always land back on Bookerly, which is Kindle's proprietary typeface. It's just pleasant to look at and easy to read.

Sans in general I'd say are best for reading but apart from that I'd say personal choice.

I like a free font called ghandi sans personally.

I almost always switch to Roboto as soon as I open a new ebook.

In my household it differs from person to person. My preference is Trebuchet white-on-black. I don't know why, but the words just melt into my brain.

Try OpenDyslexic. It's made for dyslexic people, but god I find it so comforting to read with