LaTeX: Allow to override papersize and pointsize from LaTeX themes

tests/roots/test-latex-theme/theme/custom/theme.conf

@@ -1,4 +1,6 @@
 [theme]
 docclass = book
 wrapperclass = sphinxbook
+papersize = a4paper
+pointsize = 12pt
 toplevel_sectioning = chapter

tests/test_build_latex.py

@@ -221,7 +221,18 @@ def test_latex_theme(app, status, warning):
     result = (app.outdir / 'python.tex').read_text(encoding='utf8')
     print(result)
     assert r'\def\sphinxdocclass{book}' in result
-    assert r'\documentclass[letterpaper,10pt,english]{sphinxbook}' in result
+    assert r'\documentclass[a4paper,12pt,english]{sphinxbook}' in result
+
+
+@pytest.mark.sphinx('latex', testroot='latex-theme',
+                    confoverrides={'latex_elements': {'papersize': 'b5paper',
+                                                      'pointsize': '9pt'}})
+def test_latex_theme_papersize(app, status, warning):
+    app.builder.build_all()
+    result = (app.outdir / 'python.tex').read_text(encoding='utf8')
+    print(result)
+    assert r'\def\sphinxdocclass{book}' in result
+    assert r'\documentclass[b5paper,9pt,english]{sphinxbook}' in result
 
 
 @pytest.mark.sphinx('latex', testroot='basic', confoverrides={'language': 'zh'})