According the theme documentation, in the section titled "How to change the logo" it says:
"Go to your-drupal-site-name/admin/appearance/settings/drupal8_w3css_theme and uncheck “Use the logo supplied by the theme” then upload your own website logo and click save."
What it does not say is that, unlike many of the other themes I have experimented with, W3CSS does not maintain the original dimensions of the image you uploaded.
Instead it is scaled to 110x90. Not only is this significantly smaller and loses resolution, but the image is also distorted as the aspect ratio of the original image is not maintained. (Mine was 400 x 150)
Fixed
1.6
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.