Attemp using 'dark' mode has no effect

[rxu]

Probably misusing the feature, but the story is the following.
text-formatter version: 2.11.5 (the highest possible for phpBB3).
'MEDIAEMBED_THEME' => 'dark' has been added via renderer setup.
Despite that, it doesn't seem to make any effect.
Random example link: https://www.instagram.com/p/CecpgytO2Cr is rendered still in light style. iframe tag is the following:

<iframe data-s9e-mediaembed="instagram" allowfullscreen="" loading="lazy" onload="var c=new MessageChannel;c.port1.onmessage=function(e){style.height=e.data+'px'};contentWindow.postMessage('s9e:init','https://s9e.github.io',[c.port2])" scrolling="no" style="border: 0px; height: 1085px; max-width: 540px; width: 100%;" src="https://s9e.github.io/iframe/2/instagram.min.html#CecpgytO2Cr#theme=dark"></iframe>

theme=dark anchor is there though. At the same time, phpBB cache file (like s9e_renderer_b7ca6995b823aca4a93e8e3adc5dbc41b309136a.php) still contains 'MEDIAEMBED_THEME'=>'' in the list.

Not sure where to dig deeper to debug this. Thanks.