Update lammps_theme (based on latest sphinx_rtd_theme)

doc/utils/sphinx-config/conf.py

@@ -117,7 +117,7 @@ exclude_patterns = ['_build']
 #show_authors = False
 
 # The name of the Pygments (syntax highlighting) style to use.
-pygments_style = 'sphinx'
+pygments_style = 'default'
 
 # A list of ignored prefixes for module index sorting.
 #modindex_common_prefix = []
@@ -161,7 +161,13 @@ html_logo = 'lammps-logo.png'
 # 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 = []
+html_static_path = ['_static']
+
+# These paths are either relative to html_static_path
+# or fully qualified paths (eg. https://...)
+html_css_files = [
+    'css/lammps.css',
+]
 
 # Add any extra paths that contain custom files (such as robots.txt or
 # .htaccess) here, relative to this directory. These files are copied