docs: upgrade lammps_theme based on sphinx_rtd_theme 1.2.0

doc/utils/sphinx-config/conf.py.in

@@ -35,6 +35,9 @@ LAMMPS_DOXYGEN_XML_DIR = '@DOXYGEN_XML_DIR@'
 #sys.path.insert(0, os.path.abspath('.'))
 sys.path.append(os.path.join(LAMMPS_DOC_DIR, 'src', '_ext'))
 
+# add lammps_theme
+sys.path.append(os.path.join(LAMMPS_DOC_DIR, 'utils', 'sphinx-config', '_themes'))
+
 # -- General configuration ------------------------------------------------
 
 # If your documentation needs a minimal Sphinx version, state it here.
@@ -47,6 +50,7 @@ extensions = [
     'sphinx.ext.mathjax',
     'sphinx.ext.imgmath',
     'sphinx.ext.autodoc',
+    'lammps_theme',
     'sphinxcontrib.jquery',
     'sphinxfortran.fortran_domain',
     'sphinx_tabs.tabs',