It seems to be impossible to customize back- and foreground color of the 'tools' presented as buttons in the Map Widget (e.g. Home, Zoom).
There are just a few ready-made templates, and customizing a 'Theme Color' does not seem to affect the buttons.
For some of the tools in the Map Widget (e.g. Search, Basemap), there is a workaround to add them as individual Widgets instead. Then you get full freedom to customize their color, as well as shape, position etc.
An obvious suggestion would be to make all tools (integrated within the Map Widget) available as separate widgets.
