i am having the same issue, i get a bonk when i try to switch to any other theme besides default. my config.php is writable, and the bit changes that needs to.
$Configuration['Garden']['Theme'] = 'default';
I'm having the same issues with v.184.108.40.206 - I see the snapshots of the themes and I can even preview them, but I can't apply any of them.
I am guessing the themes you already have, may be designated to older version.
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