[4.0] Fix css compile after #28615

[ciar4n]

Fixes css compile after #28615

Apply this patch and run node build.js --compile-css. No errors

[richard67]

I have tested this item ✅ successfully on 0ac283b

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/28619.}

[Quy]

I have tested this item ✅ successfully on 0ac283b

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/28619.}

[Quy]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/28619.}

[N6REJ]

I have tested this item ✅ successfully on 0ac283b

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/28619.}

[jwaisner]

I have tested this item ✅ successfully on 0ac283b

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/28619.}

[infograf768]

@wilsonge
Please merge as appveyor is not at stake

[wilsonge]

Thanks!