Add RTD theme dependency for docs build

docs/source/_static/.gitkeep

@@ -0,0 +1 @@
+# Placeholder to ensure the _static directory is tracked for Sphinx builds.

pyproject.toml

@@ -58,6 +58,7 @@ dev = [
     "pytest-mock>=3.15.1",
     # Documentation
     "sphinx==8.1.3",
+    "sphinx-rtd-theme>=3.0.0",
     "sphinx_immaterial>=0.13.8",
     "sphinx-autodoc-typehints==3.0.1",
     "sphinx-copybutton>=0.5.2",

requirements-dev.txt

@@ -105,6 +105,8 @@ sphinx==8.1.3
     #   sphinx-autodoc-typehints
     #   sphinx-copybutton
     #   sphinx-immaterial
+sphinx-rtd-theme==3.0.1
+    # via pyUSPTO (pyproject.toml)
 sphinx-autodoc-typehints==3.0.1
     # via pyUSPTO (pyproject.toml)
 sphinx-copybutton==0.5.2