Newbie to Discourse, customizing questions

Hi! I have recently installed discourse and am looking to customize it and am a little lost so would love a little bit of guidance.

I would like to be able to customize a similar look to this in the navigation and I suppose I need to do this through a theme/theme components.

Both the actual look and what is displayed, ie. the way the menus are showing up inside the grey background and the content in each, for example, conversation, topic, users, replies, views, activity in “latest”.
Topic, latest, conversations in the “our topics” tab, etc.

Also, how to change the word categories to something else (for example our topics in this example).

And how to change the + New Topic button to a customized button such as in this example.

As I’m new here, I’m not sure if I should be asking these questions separately or if it’s ok to lump them all here.

TIA!