Access to theme colors in custom styling logic

[wolframhaussig]

My usecase

I have an application with a StylableDataGridView where I use the RowPrePaint event to update the backcolor of each row based on the value in a specific column. My problem is: the StylableDataGridView_RowPrePaint implementation can get as far as ThemeRegistryHolder.ThemeRegistry.Current to get the theme but it has no access to the colors used.

Workarounds

1. My current workaround is var color = ThemeRegistryHolder.ThemeRegistry.Current.Capabilities.HasFlag(ThemeCapabilities.DarkMode) ? Color.DarkGrey : Color.Grey, but this does only work for the default themes - when the colors change this would look weird.
2. Implement the custom styling logic as a IThemePlugin: this works, as we get access to the colors in the Applymethod by accessing the AbstractTheme but it would require the reimplementation of the complete StylableDataGridView styling logic.
3. Create a custom theme implementation by subclassing AbstractTheme: We could call the parent Apply method to get the default StylableDataGridView styling and then apply custom styling. The downside is that we would need to do that for each theme that should be supported and it would break when a non-customized theme would be used.

The question

Basically, my question is: How can we provide access to the colors of a theme to custom code? The current implementation only provides access to colors for:

• custom theme implementations
• theme plugins

Every other caller only has access to the ITheme interface which only supports applying it to forms and controls (we do not want to mention casting it to an AbstractTheme here to access the colors).

Possible Solutions

Currently, I am thinking of separating colors from themes (as I already mentioned in #9):

• We would rename AbstractTheme to DefaultTheme and make it non-abstract
• FileTheme would be restructured to not be a theme but only a holder for the given colors and would be renamed to ThemeDesign
• ThemeDesign could support easy configuration by configuring the main colors only while the other colors woulds be calculated form there(like we do in DefaultLightTheme for example)
• ResourceThemeLookup and FileThemeLookup would not create a FileTheme, but a DefaultTheme with the ThemeDesign loaded from the given resource
• the ITheme interface would get a property for reading the assigned ThemeDesign object which I could use in my usecase above to do custom styling

/cc @Nockiro