D308122 regressed the height of the preonboarding splash screen because the `height` in `content.screen_style` was moved to affect the <main> screen rather than the section-main component. This patch reverts that specific change but still ensures that the asrouter-newtab-multistage wrapper height can be customized in order to keep the flexible height of that newtab message type. Differential Revision: https://phabricator.services.mozilla.com/D311142