Any chance that you accidentally removed the style and saved the (now unstyled) label changes up to your file-wide theme? That seems more likely than corruption to me. If you accidentally had a label selected along with some other object you were styling for instance.
If you correct the style for the label and save the changes to the theme, do the changes not stick?
My best guess is that you have been a bit lumpy with managing styled and theme changes along the way and now it's coming to bite you. For what it's worth, my experience so far is that you have to be very disciplined with regard to theme modifications. It's easy—particularly if you've been using FM for a while and thus styles and themes is new thinking—to forget to save style changes as you go, and then save the theme change to incorporate the style change. I am finding that the best way is to use the appearance (and, in FM13, style) inspectors to make changes, as this focuses your mind on this approach.
Thanks Mike, but I think this is unlikely because my familarity with themes is pretty good nowadays ;-)
FM Engineering told me some time ago that a Theme in FM13 can be corrupted by introducing an object from a file created with FM12 - you have to copy only the style not the object from your old to your new to avoid this.
I understand that either the css or the way the css is controlled in FM13 is different to 12 - which makes sense as 13 introduced the ability to edit themes - hence the possibility of "corruption" from 12.
It was with this fact in mind that I eventually decided that my problem was most likely corruption of the theme.
Having reverted to an earlier version of my theme yesterday everything appears now to be behaving as expected.
Look, I don't know for sure - but having worked extensively with themes my current best guess is that it was a theme corruption issue and hence my suggestion that saving ones hard won themes as new versions in a regular manner may just prove to be good practice - its only a suggestion - but I shall be doing this for sure myself.
Thanks Keywords, but I think in this case that is not the reason - please see my note to Mike just above.
Thanks for responding
"I understand that either the css or the way the css is controlled in FM13 is different to 12 - which makes sense as 13 introduced the ability to edit themes - hence the possibility of "corruption" from 12."
That makes sense. Thanks for sharing your experience.
for the record here is what I was told - in fact the issue was specifically from classic layouts - not necessarily from 12 layouts generally -
"one thing you could try to avoid it is to not copy and paste actual layout objects (which carry extra information with them) from Classic to 13 layouts. But instead just copy & paste their *styles*. For example, you could make a copy of your existing Classic layout, change its theme to a standard* 13 theme (which will replace all your custom object styles with theme defaults), and then copy and paste individual object styles from your original layout onto corresponding objects of the new layout (and then save the results as your new theme)."