Go to the page that it is on, hit clt + shift + i and it will open the developer tools for your browser (for chrome, firefox, and ie. safari as well I think).
There is an element selector tool on the developers console. You'll want to find it (top right of console for chrome) and use it to select the button.
When you select an element using that tool it will focus the html and css in the console that corresponds to that element, in the case your submit button.
You can tinker with the css in the console by changing attributes and values (padding, margin, height, width, etc).
I like to figure out what styling I need in the console. Then I copy and paste that css into my css file and target the element in the file using a class name of the elements container, or the element itself.
You'll want to get familar with this because you will find yourself in many situations developer for a cms (not just Hubspot) where this is a useful way to fix design issues.