Add CSS variable for code block background color

Now the code block background color can be used elsewhere without
worrying about the colors getting out of sync.

src/theme/ayu-highlight.css

@@ -6,7 +6,7 @@ Original by Dempfi (https://github.com/dempfi/ayu)
 .hljs {
   display: block;
   overflow-x: auto;
-  background: #191f26;
+  background: var(--code-block-bg);
   color: #e6e1cf;
   padding: 0.5em;
 }

src/theme/css/variables.css

@@ -28,6 +28,7 @@
     --links: #0096cf;
 
     --inline-code-color: #ffb454;
+    --code-block-bg: #191f26;
 
     --theme-popup-bg: #14191f;
     --theme-popup-border: #5c6773;
@@ -67,7 +68,8 @@
 
     --links: #2b79a2;
 
-    --inline-code-color: #c5c8c6;;
+    --inline-code-color: #c5c8c6;
+    --code-block-bg: #1d1f21;
 
     --theme-popup-bg: #141617;
     --theme-popup-border: #43484d;
@@ -108,6 +110,7 @@
     --links: #20609f;
 
     --inline-code-color: #301900;
+    --code-block-bg: #f6f7f6;
 
     --theme-popup-bg: #fafafa;
     --theme-popup-border: #cccccc;
@@ -147,7 +150,8 @@
 
     --links: #2b79a2;
 
-    --inline-code-color: #c5c8c6;;
+    --inline-code-color: #c5c8c6;
+    --code-block-bg: #1d1f21;
 
     --theme-popup-bg: #161923;
     --theme-popup-border: #737480;
@@ -188,6 +192,7 @@
     --links: #2b79a2;
 
     --inline-code-color: #6e6b5e;
+    --code-block-bg: #f6f7f6;
 
     --theme-popup-bg: #e1e1db;
     --theme-popup-border: #b38f6b;
@@ -228,7 +233,8 @@
 
         --links: #2b79a2;
 
-        --inline-code-color: #c5c8c6;;
+        --inline-code-color: #c5c8c6;
+        --code-block-bg: #f6f7f6;
 
         --theme-popup-bg: #141617;
         --theme-popup-border: #43484d;

src/theme/highlight.css

@@ -59,7 +59,7 @@
 .hljs {
   display: block;
   overflow-x: auto;
-  background: #f6f7f6;
+  background: var(--code-block-bg);
   color: #000;
   padding: 0.5em;
 }

src/theme/tomorrow-night.css

@@ -79,7 +79,7 @@
 .hljs {
   display: block;
   overflow-x: auto;
-  background: #1d1f21;
+  background: var(--code-block-bg);
   color: #c5c8c6;
   padding: 0.5em;
   -webkit-text-size-adjust: none;