I have dark stool sometimes, it's not black rather dark brown and not accompanied with other symptoms, and not persistent, one day i get it normal brown and the next day it's quite darker. i don't have diarrhea,blood or pain. is it serious?

It is unlikely to be serious, however, if you have any concern, you could get your stool tested for blood/occult blood to rule out bleeding in the intestine. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs. Get HPV and Covid vaccines. Wish you good health!