To get darker text you need to select a bolder text with wider lines forming the text. The blacker text you like in home page menu is a bold text. There are many different degrees of bold. Those members here at mobile read that like darker text use Calibre Edit Book or other tools to format books to use darker text. There are many threads here in Kobo Reader forum telling how they formated books without DRM to use bolder text. Kobo editor forum should also help you.
https://www.mobileread.com/forums/forumdisplay.php?f=166
This weekend I was reading this forum on my android tablet and most of it was very readable bold text but some posts was regular text that was a little harder to read.