Rather than hand coding, Webflow provides a GUI to directly add html elements to the page, css properties to the html, and the ability to visually create pages using "proper coding techniques" behind the scenes automagically. Your "header_1" should be a class in the style panel, then simply go down to the color section within the interface and set it to white... You can add an ID tag to any element as well within the interface. The production + exported code will have your .header_1 class with a background-color: #fff property created for you in your css file. No need to attempt to do this with custom inline css code...
Watch the instructional videos from the tutorials link in the top right if your not familiar with the workflow/interface yet.
Also, you can't access the "stylesheet" directly in Webflow (not yet anyways - maybe one day?)... you can however add "inline" css into the head section of your html page as noted in your post if you need to... Just make sure you wrap it in
<style>somecode</style> tags + put it in the top section of the "custom code" area within the sites management area.
You can also export the code, then directly manipulate the html, css, js files in your text editor if you need to. If you know html/css or are like myself, learning html/css - you'll be pleasantly surprised to find that it doesn't look like a bunch of computer barf
Hope that helps, good luck.