Convert Manjaro Qonos KDE to Legacy boot from UEFI

I have installed Manjaro Qonos KDE edition on an old iMac (mid 2011). As is well known, the screen brightness control does not work well in UEFI mode on this system.
Previously, i had successfully converted Ubuntu installs to BIOS mode from UEFI mode after installation using boot-repair software. But i am not able to install boot-repair from the repositories in Manjaro.
Disk is GPT.
Is there any other way of accomplishing this?

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