Thanks v-yingjl once again.
Sorry. Probably there was a lack of clarity in my previous comment.
I do understand that the current implementation of Shape visual uses the theme's dataColors.Color1 as default.
Unfortunately, the problem I have is that I cannot change the Color1 as it is a "Custom" Corporate theme.
If I change it just for this purpose, it will affect all other visuals as well.
Regarding the second part of my previous comment:
... was in a way asking why Shape visual uses 'Color1' and not the 'background' color class for the default fill.
because as per Microsoft documentation on Themes:
data colors: The list of color hexadecimal codes to use to color shapes that represent data in Power BI Desktop visuals.
color classes, such as background and firstLevelElements. These color classes set the structural colors for elements in the report, such as axis gridlines, highlight colors, and background colors for visual elements."