Update css-practice.md

There is a lack of detail regarding the need to clear the static files cache....I spend hours following this specific section of the documentation (while in Developer mode) with *all* cache disabled, with zero effect...until I clicked on "Flush Static Files Cache". There should be clearer notification regarding this additional cache needing to be cleared at the beginning of all sections that related to css customization.

Author: ryantfowler

guides/v2.0/frontend-dev-guide/css-topics/css-practice.md

@@ -75,6 +75,8 @@ To do this, they added an overriding <code>_theme.less</code> file in the <code>
 
 </pre>
 
+Next, you need to flush the static files cache. This can be done in the Admin panel under System->Cache Management->Flush Static Files Cache or via command line by deleting the contents of `<installation_directory>/pub/static` and using `php bin/magento setup:static-content:deploy`.
+
 After the Grey theme is applied, store pages will look like following:
 
 <img src="{{ site.baseurl }}common/images/css_practice.png" alt="product page when Grey applied">