Unable to change navigation bar design

Hello, I'm posting the same question again as I haven't received any feedback. Maybe this time someone will be able to help me out 🙂

Hi, a few weeks ago I've taken over creating an intranet from scratch for my organisation. Previously there was an intranet but it was very mismanaged and cluttered with all kinds of files and folders. The problem I have is that someone who was creating the previous intranet, has somehow (likely by writing code) set a template for all organisation's sites and pages so that whenever I create one, whether I choose a blank or one of microsoft's template for it, the navigation bar always looks the same, as in the background color is the same, font of tab names is the same, the highlight color when i hover over a tab is the same and also there's always the same logo, which is mistakenly linked to a wrong outside page (I also don't know how to change that). These are the things I'd like to change. The things that I can do are add new tabs to the navigation bar and change their order, however I do not see an "Edit" button on the navigation menu and I have to do that through site's settings which also is troublesome. After analyzing the site by pressing F12 I noticed that underneath this template there is a theme color that I've set for the site. The site's I'm making are all Modern Community Sites connected to one Hub Site which will be the Home site as well as a root site when all work is done. I've also contacted Microsoft Support, however after many attempts and 3 weeks of correspondence, they were not able to help me and finally redirected me here. I'll be grateful for any help or tips. Thanks in advance!