Set color-scheme: dark (partial fix for #7)

This fixes the arrows looking light on the number input for expiry, but
not the weird-looking pseudo-skeumorphic dropdown select box for the
expiry unit on Safari.

bobashare-web/static/css/bobashare.css

@@ -1,6 +1,10 @@
 :root {
     /* horizontal margin is 1em on each side */
     --max-vw: 100vw - 2em;
+    /* tell elements that they should be rendering in dark mode
+     * namely, this fixes the dropdown selects being ugly on Safari
+     */
+    color-scheme: dark;
 }
 
 /* Main document formatting */