Recommendation for Forum Software

[GrAvEsSs]

Hello friends, I want to create a forum website for a project of mine. Honestly, since I don't have a budget, I am not leaning towards paid forum software as the market for paid forum software is well-known. What I need is a customizable software that is not behind in terms of SEO. I am considering SMF, but I haven't made a decision yet. Also, I want to have a theme made for the free forum software I will install. Can I get your recommendations?