Change my default posts layout?

Hello, is there any way to change my default posts layout and design? So all I have to do is create the post and its done, instead of every time I create a post, I have to go in and adjust and tweak my header, colors, ect ect. Would save me ALOT of time lol. I am using the “Start-Up” theme. The home page looks great with that background, but when you create individual pages, it looks all wonky.