The command Ctrl+Shift+X creates an element of the form<a class="sigil_index_marker" title="...">...</a, and creating an index then causes an id attribute to be added to the element. While this is formally valid, it causes problems in Adobe Digital Editions, which handles any <a> element (even without a href attribute) as a link, displaying it in a link-like manner.
I though I had fixed this in CSS:
.sigil_index_marker {
text-decoration: none;
color: inherit;
cursor: text;
}
Now I was told that this does help in iBooks, which still shows the markers using a link color. Maybe this can be fixed by using black instead of inherit, but this is an ugly fix, and I cannot test this directly.
I wonder if Sigil could be changed to use <span> instead of <a> for the index markers. This would avoid the problem and would be better in accordance with modern HTML style. I guess the difficult part would be to create compatibility, but I would expect that simply changing any <a> in class sigil_index_marker to <span>, when opening an existing .epub, would handle this.