Your first customization
You don't need a coding background for this. A template is just the HTML of one part of your board with placeholders where the real information goes, and the safest way to learn it is to change one small thing.
Open Appearance, then Custom Templates on your board.
Pick a surface. The Post rail is a good first one: it's the member box that sits beside every post.
The editor opens with the current design already filled in. Find the word Posts and change it to Messages.
Press Save, then open any topic on your board. The rail now says Messages.
Changed your mind? Press Revert to stock and the built-in design takes over again. Your saved version stays restorable, so nothing is ever lost.
That's the whole loop: open, tweak, save, look. Everything else in Custom Templates is the same idea with more options.
Doing something real
The variables panel beside the editor lists everything the surface knows: click one and it drops into your template at the cursor. A few beloved starter projects, each a two line change:
Show where a member is from: add a From row to the post rail using the member's location, wrapped in an if-block so it hides when empty.
Bigger topic titles: in the Topic row surface, add a font size to the title link.
A character-sheet post rail, the roleplay classic: rearrange the rail entirely around your custom profile fields.
The editor checks your template while you type and tells you the moment something wouldn't parse. A template with errors can't even be saved, so experiment freely.