"Varela Round" is closer to "Arial Rounded MT Bold" if embolden is used, but this trick only works on WebKitGTK:
<match target="pattern">
<test qual="any" name="family">
<string>Arial Rounded MT Bold</string>
</test>
<edit name="family" mode="assign" binding="strong">
<string>Varela Round</string>
</edit>
<edit name="embolden" mode="assign">
<bool>true</bool>
</edit>
</match>
"Varela Round" is closer to "Arial Rounded MT Bold" if
emboldenis used, but this trick only works on WebKitGTK: