You can use the fullwidth page template or set posts without sidebar which will adjust page-layout to use the fullwidth option. This option is available in post options given below the contents editor.
You can create fullwidth pages and fullwidth blog lists. Check Help Guide pages 46 and 50
Could you check if the requested feature is in the theme demo. If it’s not then likely it’ll need customizations to get it done. Currently Helm theme doesn’t have a fullscreen page fit options. It does have fullwdith without sidebar option.