As the subject suggests, on a new installation with dark theme it is unreadable. Font is dark and small and there is no customization either for it on the appearance tab. Workaround for devs here but that should be a simple tweak for the developer for others to use.

viewtopic.php?f=67&t=137743

Any clues?

I added a (non-dev) suggestion on the other thread: try using different desktop themes. There are a wide variety that you can download them from the kde store, so try a few and see if you find one you like.

Rog131 took that a step further by identifying the image file provided by the theme for the numbers used in the timer widget (path/to/theme/widget/timer.svgz). That file can be modified using a vector image editor like Inkscape or Karbon.
Another idea if you like the colors for the timer widget from some theme but don't like the rest of the theme is that you can copy the timer image file from that theme to the similar location in the other theme that you do like. (Plasma 4 had the ability for you to mix and match elements from different themes inside System Settings, but as I understand it that was removed in Plasma 5)

I found that there are three versions of timer.svgz in the system and a preview of them in dolphin revealed what they look like. I copied the one looking good on dark themes (oxygen) to the local timer plasmoid directory and after a reboot it shows and is ticking.

Probably original devs are not considering other users when they are not supplying the required additions to the mainstream themes causing such redundant efforts. For them it is probably a matter of cp/paste. Linux non-philosophy.

I am using KDE Plasma version 5.9.5 and Frameworks version 5.31.0

Path of the existing images:
/usr/share/plasma/desktoptheme/oxygen/widgets/timer.svgz
/usr/share/plasma/desktoptheme/default/widgets/timer.svgz
/usr/share/kde4/apps/desktoptheme/oxygen/widgets/timer.svgz

I copied the oxygen timer.svgz to
/usr/share/plasma/desktoptheme/breeze-dark/ (made a widgets folder which was lacking)

Thinking about it more, you should go ahead and file a bug against the (dark) breeze theme because the default themes provided should just work without tweaking needed, so the dark version of the breeze plasma theme should override the look of the timer.
The suggestions given were assuming the current state of things providing suggestions that you can apply right now to make things work, but fixing the default theme is a better long-term solution.

That way a theme developer should consider all widgets' requirements to accommodate them. That sounds the other way around. Isn't the widget's developer subscribing to its bugs? Or is he wishing others keep improving it?

That is a good point. Where should the responsibility of making sure the widgets look good in various theme be: with the widget designer or with the theme designer?
On the one hand, the theme designer should make sure that their theme looks good when using (all of) the default/built-in widgets, on the other hand, the widget designer shouldn't necessarily expect all theme designers to test their themes with even all of the default/built-in widgets especially some of the less used/more obscure ones like the timer (apparently).

The widget designer should accommodate a certain degree of themability so that their widget blends well with whatever theme (color scheme, font selection) the user decides to use. In this case apparently the widget designer decided to accommodate that by expecting the theme to provide the images used by the widget (similar to how different themes are expected to provide different images for the analog clock). It seems like there should be other ways to accomplish the same goal which don't require the themes to know to provide separate images for those widgets.

Thus updating the built-in theme to provide the image is more of a band-aid as it only fixes it for the one built-in theme and doesn't really fix the problem when using other themes.