I think it's a good idea to collect examples of what you're going for, drawing from scenes you thought accomplished this well or art from your favorite artists.
I have a folder on my computer dedicated to snips of art I've come across online and gathered for use as references or inspiration, as well as a sub-folder for interesting lighting/shading ideas, and it's been super helpful in this!
For a romantic vibe, I think of dim, gentle lighting, with soft red or pink-tinted colored light, and splashes of brightness with dark shadows. Maybe having part of the characters' faces cast in shadow and only narrow sections illuminated for a thrilling or mysterious look, with reddish color tones for a feeling of warmth and intimacy.
That's what I think of, anyways, based on the description you gave. There are a lot of different ways you could go about lighting a romantic setting. :]
Skimmed through my collection of lighting refs and this caught my eye as a different approach to a lovely romantic scene. Work is 'In Another World' by elbenherzart on Deviantart.
The lighting is warm and gentle, conveying a sense of gentle affection and love between the two characters. I also like that they're backlit, with the rest of their faces cast in soft shadow. It creates a feeling of cozy, comfortable closeness and intimacy.
