css: Use SCSS nesting for `.screen`.

static/styles/portico/landing-page.scss

@@ -1024,7 +1024,8 @@ nav {
     }
 }
 
-.screen .line {
+.screen {
+    .line {
         width: 100%;
         height: 6px;
         margin: 8px 0px;
@@ -1073,17 +1074,17 @@ nav {
         &.orange {
             background-color: hsl(30, 42%, 77%);
         }
-}
+    }
 
-.screen .col-4 .action-block {
+    .col-4 .action-block {
         margin: 20px;
 
         .sub-block {
             margin: 0px 10px;
         }
-}
+    }
 
-.screen .top {
+    .top {
         .avatar {
             display: inline-block;
             vertical-align: top;
@@ -1103,14 +1104,14 @@ nav {
 
             background-color: hsl(0, 0%, 73%);
         }
-}
+    }
 
-.screen .navbar {
+    .navbar {
         background-color: hsl(170, 48%, 54%);
         padding: 10px;
-}
+    }
 
-.screen .center-page {
+    .center-page {
         height: 100%;
         position: relative;
 
@@ -1122,9 +1123,9 @@ nav {
 
             transition: all 0.1s ease;
         }
-}
+    }
 
-.screen .message-feed {
+    .message-feed {
         .message .content {
             margin-left: 24px;
             position: relative;
@@ -1140,6 +1141,7 @@ nav {
 
             box-shadow: 0px 0px 15px hsla(0, 0%, 0%, 0.01);
         }
+    }
 }
 
 .portico-landing.hello .screen.hero-screen {