Fix timestamp render issue for low width screens

The font-size rule specified with ems was recursively applying to the
<time> child, causing the date itself to be even smaller. I noticied
this in Safari on iOS.

assets/css/dark.css

@@ -8,6 +8,8 @@ html {
 body {
     color: #ebebeb;
     background: #121212;
+    -webkit-font-smoothing: antialiased;
+    -moz-osx-font-smoothing: grayscale;
 }
 
 header#banner a {

assets/css/main.css

@@ -9,7 +9,7 @@ html {
 body {
     font-size: 16px;
     font-size: 1.6rem;
-    font-family: 'Helvetica Neue', 'Arial', sans-serif;
+    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
     color: #313a3d;
     width: 100%;
     margin: 0 auto;
@@ -139,10 +139,11 @@ main#content header#post-header h1 {
     display: block;
     font-size: 23px;
     font-size: 2.3rem;
+    font-weight: 600;
     line-height: 1.15;
 }
 
-main#content header#post-header div {
+main#content header#post-header > div {
     display: block;
     font-size: 0.85em;
     color: #767676;

assets/css/min770px.css

@@ -27,8 +27,8 @@ main#content ul#posts {
 
 /* single.html styles */
 main#content header#post-header h1 {
-    font-size: 26px;
-    font-size: 2.6rem;
+    font-size: 24px;
+    font-size: 2.4rem;
 }
 
 main#content img {