v4: Update homepage, examples, and general docs styles

- Refreshes the look and feel of the homepage with fresh colors, new content, and simplified styles.
- Updates docs button styles
- Replaces bytesize icons with Bootstrap icons
- Adds themes callout to examples page

site/docs/4.4/assets/scss/_buttons.scss

@@ -4,14 +4,15 @@
 
 .btn-bd-primary {
   font-weight: 600;
-  color: $bd-purple-bright;
+  color: $white;
+  background-color: $bd-purple-bright;
   border-color: $bd-purple-bright;
 
   &:hover,
   &:active {
     color: $white;
-    background-color: $bd-purple-bright;
-    border-color: $bd-purple-bright;
+    background-color: darken($bd-purple-bright, 10%);
+    border-color: darken($bd-purple-bright, 10%);
   }
 
   &:focus {

site/docs/4.4/assets/scss/_content.scss

@@ -107,7 +107,6 @@
 .bd-title {
   margin-top: 1rem;
   margin-bottom: .5rem;
-  font-weight: 300;
   @include font-size(3rem);
 }
 
@@ -122,3 +121,7 @@
 
 .bd-text-purple { color: $bd-purple; }
 .bd-text-purple-bright { color: $bd-purple-bright; }
+
+.bd-bg-purple-bright {
+  background-color: $bd-purple-bright;
+}

site/docs/4.4/assets/scss/_masthead.scss

@@ -3,13 +3,19 @@
 .bd-masthead {
   position: relative;
   padding: 3rem ($grid-gutter-width / 2);
-  // background-image: linear-gradient(45deg, #fafafa, #f5f5f5);
+  background: linear-gradient(to right bottom, lighten($bd-purple-light, 16%) 50%, #fff 50%);
 
   h1 {
     @include font-size(4rem);
     line-height: 1;
   }
 
+  .lead {
+    @include font-size(1.5rem);
+    font-weight: 400;
+    color: $gray-700;
+  }
+
   .btn {
     padding: .8rem 2rem;
     font-weight: 600;
@@ -37,16 +43,38 @@
   }
 }
 
-.half-rule {
-  width: 6rem;
-  margin: 2.5rem 0;
-}
-
 .masthead-followup {
   .bd-clipboard { display: none; }
 
+  h2 {
+    @include font-size(2.5rem);
+  }
+
   .highlight {
-    padding: .5rem 0;
-    background-color: transparent;
+    @include border-radius(.5rem);
+
+    pre::-webkit-scrollbar {
+      display: none;
+    }
+
+    pre code {
+      display: inline-block;
+      white-space: pre;
+
+      &::before {
+        display: none;
+      }
+    }
   }
 }
+
+.masthead-followup-icon {
+  padding: .75rem;
+  background-image: linear-gradient(to bottom right, rgba(255, 255, 255, .2), rgba(255, 255, 255, .01));
+  @include border-radius(.75rem);
+  box-shadow: 0 .125rem .25rem rgba(0, 0, 0, .1);
+}
+
+.masthead-followup-svg {
+  filter: drop-shadow(0 1px 0 rgba(0, 0, 0, .125));
+}

site/docs/4.4/assets/scss/_nav.scss

@@ -4,7 +4,7 @@
 
 .bd-navbar {
   min-height: 4rem;
-  background-color: $bd-purple;
+  background-color: $bd-purple-bright;
   box-shadow: 0 .5rem 1rem rgba(0, 0, 0, .05), inset 0 -1px 0 rgba(0, 0, 0, .1);
 
   @include media-breakpoint-down(md) {
@@ -38,7 +38,7 @@
     .nav-link {
       padding-right: .5rem;
       padding-left: .5rem;
-      color: $bd-purple-light;
+      color: rgba($white, .85);
 
       &.active,
       &:hover {