When dark theme used, iframe card white flashes until iframe is loaded.
It is a know issue and can be very easily enhanced with the javascript code.
Example:
<iframe style=”visibility:hidden;” onload=”this.style.visibility = ‘visible’;” src=”../examples/inlineframes1.html” > </iframe>
Trick is to use onload=”this.style.visibility = ‘visible’;” and style=”visibility:hidden;”