The useful timer widget loses its appeal when the Breeze Dark theme is applied. First, it is unreadable, second the font could be user definable and third it could also let the user to hide/unhide the hours section as currently seconds can be turned on or off by the user. That is because most will probably set under hour alarms. To most developers' surprise timer widget is using images to show the time. The reasons are not clear but it is more acceptable that using simple text produces better results since handling these sorts of issues falls automatically within the responsibility of the installed and running theme. I am using Kubuntu 17.04 and found the images in three locations meaning it was supplied for these themes: default, oxygen, KDE4 oxygen. KDE now comes with more themes out of the box and if you don't check / supply the required assets, users will have problem. It shouldn't take much time to check /supply those things. This way many more hours of other users are saved which is in line with Linux philosophy.
I am not using dark theme so it is not possible to check if the problem is solved or not. There has been no comments or others confirming the issue, then I will mark it a safe status.
Individual tickets: bug 364309 - don't work in breeze dark bug 395182 - Timer widget looks too small when docked into a panel