Chrome Extension Content Script on https://chrome.google.com/webstore/

TL;DR The webstore cannot be scripted by extensions, and the flag that previously allowed you to do that (--allow-scripting-gallery) has been removed in Chrome 35.

Chrome extensions cannot execute Content scripts / insert CSS the Chrome Web Store. This is explicitly defined in the source code, at function IsScriptableURL (click on the previous link to see the full logic).

  // The gallery is special-cased as a restricted URL for scripting to prevent
  // access to special JS bindings we expose to the gallery (and avoid things
  // like extensions removing the "report abuse" link).
  // TODO(erikkay): This seems like the wrong test.  Shouldn't we we testing
  // against the store app extent?
  GURL store_url(extension_urls::GetWebstoreLaunchURL());
  if (url.host() == store_url.host()) {
    if (error)
      *error = manifest_errors::kCannotScriptGallery;
    return false;
  }

manifest_errors::kCannotScriptGallery is defined here:

const char kCannotScriptGallery[] =
    "The extensions gallery cannot be scripted.";

The error can be viewed in the background page's console when you use chrome.tabs.executeScript to inject a script in a Web Store tab. For instance, open https://chrome.google.com/webstore/, then execute the following script in the background page of an extension (via the console, for live debugging):

chrome.tabs.query({url:'https://chrome.google.com/webstore/*'}, function(result) {
    if (result.length) chrome.tabs.executeScript(result[0].id, {code:'alert(0)'});
});