All icons named icon.svg or png on pix_plugins/mod folder are directly discarded.

If both monologo and icon are provided monologo is used, but if monologo is not provided, the fallback is the default activity icon.

Checking modinfolib.php get_icon_url function it directly ask for monologo url whether it exists or not. IMHO it should test it and if monologo does not exist, fallback to icon like it does resolve_image_location on outputlib.php

The result is that we currently accept icon.png but only if they are on the activity directly. If you want to override them on the theme, you cannot.

As an example I'm attaching a custom theme that only overrides icons and it's based on boost.

If you inspect it, assign only has icon.svg. And lesson is missing: both activity icons fallback to activity default icon. Also there are 3rd party activity icons like certificate or questionnaire.

Also core_component::has_monologo_icon should check the final url to know if  the icon included is monologo or not.