Hi, kindly check again, I have updated the page with some custom style for header (since the page built with T4 Page Builder is the main content of your page only, the header and footer belongs to template part).
There is one note that, the Joomla page builder is designed to work with other templates and framework so it's color theme could change depending on the template you use so that the page you imported can match your entire website styling (font, primary color, ...) based on Bootstrap 4 variables.
I created a theme on your T4 Blank template style that matches the color theme as Stack demo site.
We are also improving the T4 Page Builder as well as T4 Blank template to make things easier for user to setup, your case really gives us direction for improvements.
Thank you and have a nice day!