2018-09-18, 12:43
On mac, to make it work add a custom keymap.xml to /Users/<your_user_name>/Library/Application Support/Kodi/userdata/ with this inside:
xml:
<keymap>
<FullscreenVideo>
<keyboard>
<left mod="alt">PlayerControl(tempodown)</left>
<right mod="alt">PlayerControl(tempoup)</right>
</keyboard>
</FullscreenVideo>
</keymap>