I've tried giving it another go, but unfortunately it didn't work. Here's what I did:
[NEW THEMES LIST ARE NOT SHOWING]
1a Copied the Theme folder to "\BATOCERA\share\recalbox\system\.emulationstation\themes"
1b Does not show up on the themes list
1c Restarted PC
1d After restart, the themes list were still the same. Only the defaults can be selected.
[THEMES ARE SHOWING, BUT WITH MISSING VISUALS AND SOUND]
2a Copied the Themes folder to "\BATOCERA\share\system\.emulationstation\themes"
2b Does show up on the themes list
2c Applied the theme, but somehow even though selected the default is still the one active
2d Restarted PC
2e After restart, the theme has changed. However not the desired theme as the only elements showing are Black Texts, White Background and no sound.