As there seem to be an error with the pom.xml for Apache FOP on maven central, just include the two offending avalon-framework packages beforehand (and we even use the newer versions, because org.apache.servicemix.bundles.fop does too).

dependencies {
    compile 'org.apache.avalon.framework:avalon-framework-api:4.3.1'
    compile 'org.apache.avalon.framework:avalon-framework-impl:4.3.1'
    compile('org.apache.xmlgraphics:fop:1.1') {
        excludes 'avalon-framework-api', 'avalon-framework-impl'
    }
}

Create or add scripts/_Events.groovy and include this. This now just takes all the Themes there (indicated by a styles.scss file there), compiles the styles.css and removes all the **/*.scss files from the target.

import com.vaadin.sass.internal.ScssStylesheet

eventCreateWarStart = { name, stagingDir ->
    new File("${stagingDir}/VAADIN/themes/").eachDir {
        def scssName = "${it}/styles.scss"
        if (new File(scssName).exists()) {
            ScssStylesheet scss = ScssStylesheet.get(scssName)
            scss.compile()
            def outFile = new File("$it/styles.css")
            outFile.write(scss.toString())
            ant.delete(dir:"${it}", includes: "**/*.scss", verbose: true)
        }
    }
}

Checks for the version in application.properties or uses the highest version (adjust BASE)

#!/bin/sh
BASE=~/apps
version=`ls -d "$BASE"/grails-* | tail -n1 | perl -ne 'print "$1" if (/-([0-9\.]+)$/)'`
if test -f "./application.properties"; then
      version=`cat ./application.properties | perl -ne 'print "$1" if (/app.grails.version=([0-9\.]+)$/)'`
fi
"$BASE"/grails-"$version"/bin/grails "$*"

session.lockInstance.lock()
try {
        // CODE
}
finally {
        session.lockInstance.unlock()
}