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This is basically hard-coded in the tf display type plugin: https://github.com/ros-visualization/rviz/blob/melodic-devel/src/rviz/default_plugin/tf_display.cpp#L564

("basically", because in case of a timeout it will turn grey...)

So there is no way for you to change that except for changing the source code.

As to why it is white in the first place, my guess is as good as any, but I'd also say due to the default background being black...