[BREAKING] Split-up css into sub-parts (#317)

* Move styles into 3 different locations
- core
- common
- hljs

* Load all stylesheets in proper order

assets/css/core/theme-vars.css

@@ -0,0 +1,38 @@
+:root {
+    --gap: 24px;
+    --content-gap: 20px;
+    --nav-width: 1024px;
+    --main-width: 720px;
+    --header-height: 60px;
+    --footer-height: 60px;
+    --radius: 8px;
+    --theme: #fff;
+    --entry: #fff;
+    --primary: rgba(0, 0, 0, 0.88);
+    --secondary: rgba(0, 0, 0, 0.56);
+    --tertiary: rgba(0, 0, 0, 0.16);
+    --content: rgba(0, 0, 0, 0.88);
+    --hljs-bg: #1c1d21;
+    --code-bg: #f5f5f5;
+    --border: #eee;
+}
+
+.dark {
+    --theme: #1d1e20;
+    --entry: #2e2e33;
+    --primary: rgba(255, 255, 255, 0.84);
+    --secondary: rgba(255, 255, 255, 0.56);
+    --tertiary: rgba(255, 255, 255, 0.16);
+    --content: rgba(255, 255, 255, 0.74);
+    --hljs-bg: #2e2e33;
+    --code-bg: #37383e;
+    --border: #333;
+}
+
+.list {
+    background: var(--code-bg);
+}
+
+.dark.list {
+    background: var(--theme);
+}