Update docs theme to match marshmallow (#1026)

Author: sloria

docs/_static/custom.css

@@ -0,0 +1,17 @@
+/* Headings */
+
+h2, h3, h4, h5, h6 {
+  font-weight: 400;
+}
+
+/* UI elements: left and right sidebars, "Back to top" button, admonitions, Copy button */
+.sidebar-drawer, .toc-drawer, .back-to-top, .admonition, .copybtn {
+  /* Sans-serif system font stack */
+  font-family: -apple-system, BlinkMacSystemFont, avenir next, avenir, segoe ui, helvetica neue, helvetica, Cantarell, Ubuntu, roboto, noto, arial, sans-serif;
+}
+
+/* Hide ToC caption text within the main body (but leave them in the side-bar). */
+/* https://github.com/hynek/structlog/blob/b488a8bf589a01aabc41e3bf8df81a9848cd426c/docs/_static/custom.css#L17-L20 */
+#furo-main-content span.caption-text {
+  display: none;
+}

docs/conf.py

@@ -30,4 +30,18 @@
 
 # THEME
 
-html_theme = "sphinx_rtd_theme"
+html_theme = "furo"
+html_theme_options = {
+    "source_repository": "https://github.com/marshmallow-code/apispec",
+    "source_branch": "dev",
+    "source_directory": "docs/",
+    "light_css_variables": {
+        "font-stack": "Charter, Iowan Old Style, Palatino Linotype, Palatino, Georgia, serif;",
+    },
+    "top_of_page_buttons": ["view", "edit"],
+}
+pygments_dark_style = "lightbulb"
+html_static_path = ["_static"]
+html_css_files = ["custom.css"]
+html_copy_source = False
+html_show_sourcelink = False

pyproject.toml

@@ -50,7 +50,7 @@ docs = [
   "marshmallow>=3.18.0",
   "pyyaml",
   "sphinx-issues",
-  "sphinx-rtd-theme",
+  "furo",
   "sphinx>=8.1",
 ]
 tests = ["openapi-spec-validator", "pytest"]