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What is this https://www.phpbb.com/customise/db/styl ... 30303c42e7??!! I have no clue what I would look for!!! Any idea?

That is an extension. Install it to your forum, you'll be able to change the logo using an interface in the ACP after it's installed

The extension for logo install you are linking to looks very nice and chock full of features.
...but it doesn't work at least for the pro-silver dark.
It does however block the generic one except when you refresh the page and the generic one flickers for a fraction of a second.
There's a button on the extension to remoave the generic one but it doesn't work very well

add: I was able to resolve the flickering by removing the generic logo.svg since the remove original logo feature in the extension is not fully functioning. When I did this, the new logo did show up
all else with the extension seems to be functional tho - thanks for the extension link

The flickering with that Site Logo extension annoyed me too much, and made it look like there was something wrong with my forum. My solution was to create a new "colour" css file (duplicate the colour you plan to use for your board), then set the header to use that file instead of any of the default ones. It seems to be loaded last, so css rules in there seem to have priority over any in the other files, so I just add to it any css tweaks I want to make to the style, including my custom site logo:

.site_logo {
background-image: url("/images/<logo_name>.png");
}

It seems to work fine without any issues and makes it a bit easier to include any custom tweaks in future theme updates, since they're all in that one custom file.

Also, I've only just switched to this theme recently, so thank you for it! I've made a few custom tweaks for the text colour and font, using the above custom css file, and it's looking great