Here is documentation on starting your own theme: https://docs.nodebb.org/development/themes/
If you want to make small style changes, it's best to do it in the custom CSS portion of the ACP, instead of editing the theme files directly.
the logo above the forum categories shows on Chrome Desktop, but does not show anymore on Chrome for Android (Samsung S8) nor iPhone S5 with Safari. On the mobile phones, the logo is in fact shown, but the blue top bar with the three dashes and the chat icon are on top of it.
Before the logo was shown nicely below the blue top bar, and it seems there is a bug .. On the iPhone you can "pull down" the webpage a bit, then the logo can be seen (but it slides up again). On Android we do not have this pull down feature, so it looks like the logo is not loaded, but it is, just hidden.