UPDATE: Issue fixed with new json theme. However, mattlee there is a problem with the button fill. The fill doesn't actually fill the whole of the last button and is a visual glitch. See the screensh...
mattlee
Microsoft Employee
Microsoft EmployeeStatus changed:
Accepted
to Accepted
A second issue related to conditional formatting rules for New Slicer (Preview) with May Desktop has been identified, we plan to QFE Desktop within a few days so you can continue to author new format rules for Button and List Slicer in the May release. Existing saved rules are fine. This issue is not related to custom themes. If you save a file with conditional format rules for New Button or List slicer, you may encounter a silent upgrader error, which will cause the formatting to be lost the next time you open the file. The Desktop QFE will fix this for May soon, but Edit/Save in the web will take longer to be patched.