Ok, the best way to make your modifications and also future proof upgrading your site:
yourthemename = anyname you like
Copy the style/whitebluegrey.css to styles/yourthemename.css
If the style has a folder copy that folder to yourthemename folder keeping the contents
Now change to the templates/styles folder
and copy the whitebluegrey folder to a yourthemename folder copying the contents as well.
Now you have a the basis of your own theme in Tiki.
Under Admin->general your theme should now be listed as yourthemename.css select it and save.
Now, if you upgrade the tiki version you can keep all your modifications in tact within your theme.
Incedentally if you want to change any of the tpl templates files in templates/ copy them to templates/styles/yourthemename and work on that copy.
any tpls in the templates/styles subfolders take precidence over any in the base templates/ folder
a common example if tiki-top_bar.tpl or tiki-bot_bar.tpl
Hope I explained that lot ok