Improve README about default VS Code theme

[thernstig]

This will get you the bleeding-edge syntax highlighting for C++. Which means your theme will be able to color your code better. This used to be a fix, but then VS Code starting using it as the official source for C and C++ highlighting.

Does This used to be a fix refer to that this extension used to be a fix? And it is not needed anymore if using the default C/C++ extension?

NOTE: The default VS Code theme does not color much. Switch to the Dark+ theme (installed by default) or use a theme like one of the following to benefit from the changes:

Does default here mean the default one called Dark Modern?
Does this text imply that even if I install this extension (Better C++ Syntax) I still need to use Dark+ i.e. that the default Dark Modern does not work even with this extension?

[thernstig]

Has anyone got an idea if the text This used to be a fix means this extension is not necessary anymore? It sounds like the C/C++ extensions uses the highlighting features, that this extension here brings, by default now.