How to edit rocket chat html files and edit main view

Hello , am new to meteor framework ,
i have to task to add an icone beside the 7 icons already exist in the left side bar ,
and it would produce a list menu ,

but i have no idea at all where to edit the files , i cant find any html file to edit , am an express guys ,
can some please help me , to figure this out , thanks in advance .

I would also like an example for this one. Would love for example to be able to add a link to the sidebar. I want to link to a manual or a payment gateway, or maybe display advertisements in the gui.