Well… I checked this a bit deeper now.
My original logo file (Logo – Lumen Vitae) is 1600 x 481.
The theme has it cropped (cropped-Logo.png – Lumen Vitae) and the cropped image is 665 x 200.
So, the ratio is the same (well, almost the same - I guess you cannot get better with integers). That is not surprising since upon cropping I have selected the entire image (see also How to skip cropping logo and icon images? for a related topic).
Why the cropped logo has width 665? I presume this is what the theme considers “full width”. Indeed, inspecting the page (https://lumen-vitae.online/) in the browser shows that the logo is shown at that exact width of 665px. I wonder why it is 665 when the page is actually wider. The slider image is 1069px wide… But never mind, I assume this is the way it was intended to be - very well.
So, I guess I could use the same for max-width of the SVG image, right? Is my reasoning valid here?
Can this 665px be expressed somehow by a named constant or something? To avoid “magic value” that would be a maintenance issue - if the theme with an update would change this size for some reason I would need to update the manual entered 665px.