Move global styles to global CSS file

New versions of Next.js aren't working with <styled jsx global> well.
Instead of debugging, this commit moves those global styles to our
global.css file.

Also changes around some border colors in dark mode to reduce their
prominence.

Tested by poking around in dev mode and release mode.

Author: jil-stripe

components/Shell/Footer.js

@@ -139,7 +139,7 @@ export function Footer({ children: links, landing }) {
             display: block;
             width: 100%;
             border: none;
-            border-bottom: 1px solid var(--dark);
+            border-bottom: 1px solid var(--page-border);
           }
 
           .try-link {

components/Shell/TopNav.js

@@ -85,7 +85,7 @@ export function TopNav({ children }) {
             margin: 0 auto;
             align-items: center;
             justify-content: space-between;
-            border-bottom: 1px solid var(--dark);
+            border-bottom: 1px solid var(--page-border);
             padding: 1rem 2rem 1.1rem;
             font-size: 15px;
             font-family: var(--sans);