I'm also getting the warning. For some reason it's trying to find the css file under default directory.
The final generated html file has good link:
<link href="/MoneyTracker-1.0-SNAPSHOT/faces/javax.faces.resource/css/icons.css?ln=bsf" rel="stylesheet" type="text/css">
When I copy file from mavenResources/META-INF/resources/bsf/css/icons.css
to mavenResources/META-INF/resources/bsf/css/default/icons.css
in the source of BootsFaces the warning goes away, but it is not the correct fix, I'm pretty sure.
That is, I think there is some problem in configuration of BootsFaces.