Be yourself; Everyone else is already taken.— Oscar Wilde.
The great thing about romance is that it’s in everything. So whether you are partial to scf-fi stories, westerns, or historicals you can write romance into any of those genres. You can prefer tones of comedy, drama, or thriller and you can find romance.
The only thing you have to include is a HEA – Happily Ever After for the protagonist. Readers pick up romances spefically expecting to follow a character (or two if you include the love intrests POV) through this adventure, struggle, whatever and end up in the arms of the one they love.
Period. End of Story. If your not ending it with that, then you aren’t writing a romance. You could be writing a fantastic tragedy or coming of age, but your aren’t writing romance.