I am not sure how to help you the best. I see the guestbook buttons are styled the same as the contact form buttons.
It seems to be a setting in your theme. Did you already check Appearance > Customizer for your form and button styling?
I could paste some better CSS in here, but it might be the wrong way to help. If needed, your question might be better placed at the support forum of oceanwp. They will know what the settings are and how you can change them.
If I see it correctly, standard color for the button is blue with white text. Somehow you ended up with black buttons and blue text.