Please refer to the codex article – Theme Changes for 5.4.2 – and see the CSS section on the ui-dialog. These changes became necessary because of changes to WordPress CSS on their 3.9 release.

Yellow Swordfish said
Please refer to the codex article – Theme Changes for 5.4.2 – and see the CSS section on the ui-dialog. These changes became necessary because of changes to WordPress CSS on their 3.9 release.

So how do I fix my forum so it works as it did before this update?

Add the changes in the codex article that Andy linked to into your custom sp theme…  unfortunate when wp changes common use css and we have to update like this…

Thank you, I followed the tutorial and everything works as it did before.

Good news. Thanks for the update.

the next version of sp should feature child themes…  in that way, you can have a minimal set of changes to a core theme and still get benefits of required updates like this one…   I already have a rough working draft of the child themes implemented and working…  but no eta on a release…