You need to place all resources in a folder ‘public’, which you need to create in projectname/themesource/[modulename]/. You can add the standard folders recourses and in it fonts and images.
You can then refer to url('./resources...) in your scss files to find them.
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To solve the issue, all the resources need to be in projectname/themesource/modulename/public. You need to create the folder public yourself. In public I added a folder resources and in this folder I added a folder for images and a folder for fonts.
In my scss file in themesource/myfirstmodule I use:
.backgroundImage{
background: url("./resources/images/login_background.jpg") no-repeat;
background-size: cover;
}
for my fonts that I use in the scss file in theme/web I use the path
url('./resources/fonts/FontName.woff2')
Just by using url('./resources...) Mendix finds my resources.