Hi Tuan,
I'm using a lightbox for the contact form and having trouble modifying the color / size of the lightbox button to submit the form and the "X" that closes the lightbox itself. Tried injecting your code into Design > Custom CSS.
At the very bottom of this page:
https://abovesummitdraft.squarespace.com/
"Let's Go" opens the lightbox form and the button I can't modify is the "get started" button on the form.
Thanks!