Add dark mode to API Docs (#24971)

Add a dark mode to Swagger UI via CSS `invert`. No toggle or anything,
but I think it's better than nothing. Users can toggle via their OS.

Also includes a few misc CSS cleanups on the page.

<img width="1264" alt="Screenshot 2023-05-28 at 20 25 06"
src="https://github.com/go-gitea/gitea/assets/115237/de761b85-ca0c-4220-bee4-73798a4360a0">
<img width="1260" alt="Screenshot 2023-05-28 at 20 02 54"
src="https://github.com/go-gitea/gitea/assets/115237/29188ed2-c167-47f5-bf28-46193d1da22c">

web_src/css/standalone/swagger.css

@@ -1,12 +1,7 @@
-html {
-  box-sizing: border-box;
-  overflow-y: scroll;
-}
-
 *,
 *::before,
 *::after {
-  box-sizing: inherit;
+  box-sizing: border-box;
 }
 
 body {
@@ -24,8 +19,24 @@ body {
   align-items: center;
 }
 
+.swagger-back-link:hover {
+  text-decoration: underline;
+}
+
 .swagger-back-link svg {
   color: inherit;
   fill: currentcolor;
   margin-right: 0.5rem;
 }
+
+@media (prefers-color-scheme: dark) {
+  body {
+    background: #1e1e1e;
+  }
+  .swagger-ui, .swagger-back-link {
+      filter: invert(88%) hue-rotate(180deg);
+  }
+  .swagger-ui .microlight {
+      filter: invert(100%) hue-rotate(180deg);
+  }
+}