I'm creating the header for an SXA site. I created a new
header Partial Design, and then in Experience Editor added some stuff from SXA toolbox. So now, how do I apply (my custom) header CSS classes for the entire header partial desgin?
There are two ways to style your component/page/partial design:
Export the page (on which you've used the partial design) using creative exchange ZIP export and apply your CSS class to HTML tags and implement rules in style.css. https://doc.sitecore.com/developers/sxa/17/sitecore-experience-accelerator/en/change-a-site-design-using-creative-exchange.html
Create styles in Sitecore for the components in your header partial design. https://doc.sitecore.com/developers/sxa/17/sitecore-experience-accelerator/en/add-a-style-for-a-rendering.html