I now use full black text on a slight grey background:
Background: R=204, G=204, B=204 ; #CCCCCC
Text: R=0, G=0, B=0 ; #000000
as set up in both PocketBook (which is my main app) and Moon+ Reader Pro.
Note that these may render slightly differently on different displays.
https://www.rapidtables.com/web/color/RGB_Color.html
These colors work for me from bright interior lighting down to pitch black interior. {EDIT: I think many may find this background too light as a result of the common practice of users having brightness set far too high on displays, instead of letting their eyes adapt.}
I don't read much in the sun (I already have the scars where lesions have been cut out caused by too much time in the sun) but when I do I have no problem reading with those colors; however I do know how to position a display so that reflected light is of little problem.
I used to use a background with some yellow in it to give a slightly aged parchment like or very light beige color but as the reading apps in more recent times have made it so easy to adjust light level from practically completely dark to bright (the finger gesture Leebase mentions) I have discarded that background.