Contact Form fields span entire page width

I am using the latest grav skeleton one page modular site as the basis for my site. The form I have added displays fields the full width of the page. I have tried styling the form with custom.css to no avail. I have been reading all day about theming but playing with twig templates seem like a bridge too far for my current level of expertise. How do I go about making my form fit say 75% of the screen in a responsive manner, or to simply fit in well with the rest of the modular page hero and feature module widths. I feel I have missed something that should be built in - simply a class in my form frontmatter maybe - a template that encapsulates my form on the page!?? I have the form plugin installed BTW.

add the form to your page via template, define a CSS class in your template should work, IMHO.

Thanks hoernerfranz for your HO - I appreciate it and hope you can give me just a little more detail: I have noticed that quark has no templates/form.html.twig so I assume the form is rendering based on plugins/form/templates/form.html.twig although I don't understand how. So my plan is derive a custom theme based on quark and add a templates/form.html.twig template to it along with a CSS file. Should this reference the plugins/form/templates/form.html.twig somehow to render the form within my theme's form template? Can I copy a form.html.twig from another theme or rather try to derive it from quark's text.html.twig say?