Removed hardcoded style from design settings index

no issue

- by using flex display on the outer container we can get rid of the hardcoded height calc style and let the browser handle the layout more naturally

ghost/admin/app/templates/settings/design/index.hbs

@@ -1,4 +1,4 @@
-<section class="gh-canvas relative w-100">
+<section class="gh-canvas flex flex-column h-100">
     <GhCanvasHeader class="gh-canvas-header">
         <h2 class="gh-canvas-title">{{this.config.blogTitle}}</h2>
         <section class="view-actions">
@@ -18,7 +18,7 @@
         </section>
     </GhCanvasHeader>
 
-    <div style="height: calc(100% - 96px)">
+    <div class="view-container">
         <GhBrowserPreview @icon={{this.settings.icon}} @title={{this.config.blogTitle}}>
             <GhHtmlIframe id="site-frame" class="site-frame gh-branding-settings-preview" @html={{this.themeManagement.previewHtml}} />
         </GhBrowserPreview>