Numerous performance fixes.

- Refactored themes to always load with just a single file. This avoids
  a round trip on each request.
- Locally load "Fira Code" when available on the system, then try to
  load the woff2 file, then fall back to regular woff.
- Update markup to use relative urls where it is safe. RSS feeds still
  use absolute URLs.
- Only set og:image:(height/width) when using the standard color icon.
- Fixed styles on the pagination buttons.

assets/css/color/blue.css

@@ -1,10 +1,4 @@
-@import '../main.css';
-@import '../header.css';
-@import '../logo.css';
-@import '../pagination.css';
-@import '../post.css';
-@import '../syntax.css';
-@import '../variables.css';
+@import '../style.css';
 
 :root {
   --accent: #23B0FF;

assets/css/color/green.css

@@ -1,10 +1,4 @@
-@import '../main.css';
-@import '../header.css';
-@import '../logo.css';
-@import '../pagination.css';
-@import '../post.css';
-@import '../syntax.css';
-@import '../variables.css';
+@import '../style.css';
 
 :root {
   --accent: #78E2A0;

assets/css/color/orange.css

@@ -0,0 +1 @@
+@import '../style.css';

assets/css/color/pink.css

@@ -1,10 +1,4 @@
-@import '../main.css';
-@import '../header.css';
-@import '../logo.css';
-@import '../pagination.css';
-@import '../post.css';
-@import '../syntax.css';
-@import '../variables.css';
+@import '../style.css';
 
 :root {
   --accent: #EE72F1;

assets/css/color/red.css

@@ -1,10 +1,4 @@
-@import '../main.css';
-@import '../header.css';
-@import '../logo.css';
-@import '../pagination.css';
-@import '../post.css';
-@import '../syntax.css';
-@import '../variables.css';
+@import '../style.css';
 
 :root {
   --accent: #FF6266;