improve css (#615)

docs/_static/css/custom.css

@@ -0,0 +1,3 @@
+pre {
+    white-space: break-spaces;
+}

docs/conf.py

@@ -94,6 +94,12 @@ html_context = {
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-html_static_path: list = []
+html_static_path = ["_static"]
+
+# These paths are either relative to html_static_path
+# or fully qualified paths (eg. https://...)
+html_css_files = [
+    "css/custom.css",
+]
 nb_execution_mode = "off"
 myst_enable_extensions = ["colon_fence"]