WoltLab Suite Core 5.3 upgrades the bundled SCSS compiler from
leafo/scssphp 0.7.x to
With the updated composer package name the SCSS compiler also received updated namespaces.
WoltLab Suite Core adds a compatibility layer that maps the old namespace to the new namespace.
The classes themselves appear to be drop-in compatible.
Exceptions cannot be mapped using this compatibility layer, any
catch blocks catching a specific Exception within the
Leafo namespace will need to be adjusted.
More details can be found in the Pull Request WoltLab/WCF#3415.