got this error when using toolbox-loader to load theme scss file replacement.
Invalid CSS after "...building: true;": expected 1 selector or at-rule, was "@mixin font-face($"
there's an extra character in front of "@mixin font", checked the original file, nothing there. looks like fs.readfile added it. added this code does the fix:
theme = theme.replace(/^\uFEFF/, '');
it's on windows platform, though. any suggestion on fixing it without code change?