Fix #2597 — allow disabling code.css genereation

Signed-off-by: Chris Warrick <kwpolska@gmail.com>

CHANGES.txt

@@ -4,6 +4,8 @@ New in master
 Bugfixes
 --------
 
+* If ``CODE_COLOR_SCHEME`` is empty, don’t generate ``code.css``
+  (Issue #2597)
 * Don’t warn about ``nikolademo`` DISQUS account when comments are
   disabled (Issue #2588)
 * Make ``data`` from global context available to templated shortcodes

nikola/conf.py.in

@@ -710,7 +710,7 @@ IMAGE_FOLDERS = {'images': 'images'}
 # SHOW_INDEX_PAGE_NAVIGATION = False
 
 # Color scheme to be used for code blocks. If your theme provides
-# "assets/css/code.css" this is ignored.
+# "assets/css/code.css" this is ignored. Leave empty to disable.
 # Can be any of:
 # algol
 # algol_nu

nikola/plugins/task/copy_assets.py

@@ -79,7 +79,7 @@ def gen_tasks(self):
                 yield utils.apply_filters(task, kw['filters'])
 
         # Check whether or not there is a code.css file around.
-        if not code_css_input:
+        if not code_css_input and kw['code_color_scheme']:
             def create_code_css():
                 from pygments.formatters import get_formatter_by_name
                 formatter = get_formatter_by_name('html', style=kw["code_color_scheme"])