It sounds like you are just deleting your temporary internet files/cookies?
"Purging your cache" means purging your phpBB cache on your server. If you change style elements, like HTML or CSS files then phpBB will continue to serve up cached copies of the old versions until you purge the cache.
See here for instructions: https://www.phpbb.com/support/docs/en/3 ... pbb-cache/
Trying doing that and report back.