Use dark-themed swagger docs

src/api.py

@@ -1,10 +1,12 @@
 from asyncio.exceptions import CancelledError
 from collections.abc import AsyncIterator
 from contextlib import asynccontextmanager
+from typing import cast
 
 from fastapi import FastAPI
+from fastapi.openapi.docs import get_swagger_ui_html
 from fastapi.requests import Request
-from fastapi.responses import PlainTextResponse
+from fastapi.responses import HTMLResponse, PlainTextResponse
 
 from src.utils.logging import get_logger
 
@@ -39,6 +41,7 @@ app = FastAPI(
         "email": "p_vacho@utb.cz",
     },
     license_info={"name": "GPL-3.0-or-later"},
+    docs_url=None,
 )
 
 
@@ -46,3 +49,33 @@ app = FastAPI(
 async def ping(_r: Request) -> PlainTextResponse:
     """Test whether the API works."""
     return PlainTextResponse("pong")
+
+
+@app.get("/docs", include_in_schema=False)
+async def docs() -> HTMLResponse:
+    """Custom endpoint to serve the Swagger UI documentation, but in dark theme.
+
+    Light theme should be considered a crime against humanity.
+    """
+    html_resp = get_swagger_ui_html(
+        openapi_url=cast(str, app.openapi_url),
+        title=app.title + " - Swagger UI",
+        oauth2_redirect_url=app.swagger_ui_oauth2_redirect_url,
+        swagger_js_url="https://unpkg.com/swagger-ui-dist@5/swagger-ui-bundle.js",
+        swagger_css_url="https://unpkg.com/swagger-ui-dist@5/swagger-ui.css",
+    )
+
+    # Swagger UI doesn't support dark theme out of the box
+    # https://github.com/swagger-api/swagger-ui/issues/5327
+    # Extract the HTML content from the default response and inject a dark mode CSS
+    html = cast(bytes, html_resp.body).decode()
+    dark_mode_css_url = "https://cdn.jsdelivr.net/gh/jcphlux/swagger-ui-themes@master/dist/swagger-dark-modern-ui.css"
+    # The hash can be generated with: openssl dgst -sha384 -binary style.css | openssl base64 -A
+    dark_mode_css_hash = "sha384-LUPDO6OZKHtaY6oPBOnbv5cmmD7ldzmDWHJe71Jlt+8+0YOTOLRzMkmUIfvVtYe+"
+    dark_mode_css_url = (
+        f'<link rel="stylesheet" type="text/css" href="{dark_mode_css_url}" '
+        f'integrity="{dark_mode_css_hash}" crossorigin="anonymous">'
+    )
+    html = html.replace("</head>", f"{dark_mode_css_url}</head>")
+
+    return HTMLResponse(html)