Button styles are theme settings, so you do not need custom CSS for them. Open any page in the page editor, click Global theme styles in the left panel and go to Buttons. Focus Plus has three button styles, Primary buttons, Secondary buttons and Tertiary buttons, and each one has its own Text, Background, Border, Corner, Shadow, Size, Hover styles and Disabled groups. The free Focus theme has the Primary buttons group only.
Form submit buttons and HubSpot CTAs follow one of these three styles: choose which one under Forms > Button > Button type and Buttons > CTA buttons > Button type. Click Publish changes at the top right when you are done; theme settings apply to every page.