docs: use custom fonts

docs/help/contents/_include/custom.css

@@ -1,8 +1,121 @@
+/* open-sans-regular - vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic */
+@font-face {
+  font-family: "Open Sans";
+  font-style: normal;
+  font-display: swap;
+  font-weight: 400;
+  src: local(""),
+    url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-regular.woff2")
+      format("woff2"),
+    /* Super Modern Browsers */
+      url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-regular.woff")
+      format("woff"),
+    /* Modern Browsers */
+      url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-regular.ttf")
+      format("truetype");
+}
+/* open-sans-600 - vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic */
+@font-face {
+  font-family: "Open Sans";
+  font-style: normal;
+  font-weight: 600;
+  font-display: swap;
+  src: local(""),
+    url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-600.woff2")
+      format("woff2"),
+    /* Super Modern Browsers */
+      url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-600.woff")
+      format("woff"),
+    /* Modern Browsers */
+      url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-600.ttf")
+      format("truetype");
+}
+/* open-sans-700 - vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic */
+@font-face {
+  font-family: "Open Sans";
+  font-style: normal;
+  font-weight: 700;
+  font-display: swap;
+  src: local(""),
+    url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-700.woff2")
+      format("woff2"),
+    /* Super Modern Browsers */
+      url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-700.woff")
+      format("woff"),
+    /* Modern Browsers */
+      url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-700.ttf")
+      format("truetype");
+}
+/* open-sans-italic - vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic */
+@font-face {
+  font-family: "Open Sans";
+  font-style: italic;
+  font-weight: 400;
+  font-display: swap;
+  src: local(""),
+    url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-italic.woff2")
+      format("woff2"),
+    /* Super Modern Browsers */
+      url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-italic.woff")
+      format("woff"),
+    /* Modern Browsers */
+      url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-italic.ttf")
+      format("truetype");
+}
+/* open-sans-600italic - vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic */
+@font-face {
+  font-family: "Open Sans";
+  font-style: italic;
+  font-weight: 600;
+  font-display: swap;
+  src: local(""),
+    url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-600italic.woff2")
+      format("woff2"),
+    /* Super Modern Browsers */
+      url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-600italic.woff")
+      format("woff"),
+    /* Modern Browsers */
+      url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-600italic.ttf")
+      format("truetype");
+}
+/* open-sans-700italic - vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic */
+@font-face {
+  font-family: "Open Sans";
+  font-style: italic;
+  font-weight: 700;
+  font-display: swap;
+  src: local(""),
+    url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-700italic.woff2")
+      format("woff2"),
+    /* Super Modern Browsers */
+      url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-700italic.woff")
+      format("woff"),
+    /* Modern Browsers */
+      url("/fonts/open-sans-v34-vietnamese_latin-ext_latin_hebrew_greek-ext_greek_cyrillic-ext_cyrillic-700italic.ttf")
+      format("truetype");
+}
+
+/* fira-code-regular - latin */
+@font-face {
+  font-display: swap; /* Check https://developer.mozilla.org/en-US/docs/Web/CSS/@font-face/font-display for other options. */
+  font-family: "Fira Code";
+  font-style: normal;
+  font-weight: 400;
+  src: url("/fonts/fira-code-v21-latin-regular.woff2") format("woff2"),
+    /* Super Modern Browsers */ url("/fonts/fira-code-v21-latin-regular.woff")
+      format("woff"),
+    /* Modern Browsers */ url("/fonts/fira-code-v21-latin-regular.ttf")
+      format("truetype"); /* Safari, Android, iOS */
+}
+
 html.light {
+  --font-family: "Open Sans", sans-serif;
+  --monospace-font-family: "Fira Code", monospace;
   --primary: #008837;
   --header-fg: #008837;
   --link: #008837;
   --heading: #000;
+  --page-bg: #f7f7f7;
 }
 
 html.dark {