For manhwa with relationship buildup and no dark themes:
-Heesu in Class 2
-Tu Quoque
-Till Our Lips Touch
-Path to You
-Behind the Desks (there is a side character who is obsessive/psychologically abusive, but not the main couple)
-Incidentally Dyed by Spring's Love (the relationship doesn't start the healthiest way because the main characters were both dealing with stuff (poverty, abandonment issues & death of a loved one) that they didn't talk to the other about, but it gets better once they start communicating and become more brave about their feelings)
Also recommending this because it's by the same artist as this manhwa (Upi) so the art's amazing, it does have dark themes like mental health issues, death (by cancer and suicide), and obsession, but it is a similar vibe and the main couple ends up in a very supportive and loving relationship:
-The Last Stop on the Way