Check the less "imports" in your custom themes. They all must match the pattern @import 'foo/_bar.less';

Pay attention at two things:

1. File path must be wrapped in double quots ("), not in single qouts (')
2. File path must end with ".less"

Check the less "imports" in your custom themes. They all must match the pattern @import 'foo/_bar.less';

Pay attention at two things:

1. File path must be wrapped in single quotes ('), not in double quotes (")
2. File path must end with ".less"