Hi Urs,
I tested this with minor changes to lib/custom_corners_lib.php, lib/weblib.php and theme/custom_corners/user_styles.css
I changed
<div class="wrap">
<div class="bt"><div></div></div>
....
<div class="bb"><div></div></div>
</div>
to a "three image box" - left image, border image and right image for top and bottom of the box using
<DIV id="box">
<DIV id="box_t">
<DIV id="box_tl">
<DIV id="box_tr"></DIV></DIV></DIV>
....
<DIV id=box_b>
<DIV id=box_bl>
<DIV id=box_br></DIV></DIV></DIV>
</DIV>
It worked fine with both rtl and ltr languages and without a need to use styles_rtl.css at all or any other fixes. I will test some more (first I will try the order tl-t-tr...bl-b-br with some new css because I did not test that yet) and send you some mail later...
I am working on the rtl issue. Hope to find a flexible solution. You may look at the linked issue.