Comment to Default cover page and icon for categories?
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There is no separate page for browsing content in category, it just shows search result page. However for the tags it's possible, you can install Channels module, then every hashtag will have separate channel, and you as admin can change cover in the channel. Additionally you can use hashtags as labels (like in this forum).
Colour of user icon is generated automatically, if you need the same colour for all users you can override via CSS.