Tweak link/code styles (#435)

- Tone down link background
- Tone down code background
- Shrink font -- I'd prefer 15px Menlo and 17px Consolas but we'll meet
in the middle at 16px where both look reasonable, unlike 17px Menlo
which was comically large.

![before-after](https://user-images.githubusercontent.com/6820/34066487-360cf57e-e1c4-11e7-891e-db5d4cd59b1d.gif)

src/theme.js

@@ -98,7 +98,7 @@ const fonts = {
 // Except when they must be used within nested CSS selectors.
 // This is the case for eg markdown content.
 const linkStyle = {
-  backgroundColor: hex2rgba(colors.brandLight, 0.5),
+  backgroundColor: hex2rgba(colors.brandLight, 0.3),
   borderBottom: `1px solid ${hex2rgba(colors.black, 0.2)}`,
   color: colors.text,
 
@@ -233,10 +233,13 @@ const sharedStyles = {
     },
 
     '& p > code, & li > code': {
-      background: hex2rgba(colors.note, 0.3),
-      padding: '0 3px',
-      fontSize: 'inherit',
+      background: hex2rgba(colors.note, 0.2),
       color: colors.text,
+    },
+
+    '& p > code, & li > code, & p > a > code, & li > a > code': {
+      padding: '0 3px',
+      fontSize: 16,
       wordBreak: 'break-word',
     },