Missing properties from system.theme.css in Basis

[stpaultim]

Description of the bug
The option to place the field label above the input field does not work in Webform because Basis excludes the system.theme.css file. backdrop-contrib/webform#152

Webform seems to expect the following:

label {
display: block;
font-weight: bold;
}

Basis does set:

label {
    font-weight: 600;
}

I assume that system.theme.css was excluded for a reason, so I'm not sure if its correct to add this to Basis theme or if the correct solution is for Webform to add this itself:

label {
display: block;
}

The argument in favor of adding this to Basis is that if Webform is expecting this to be provided by the system, maybe other modules are as well. Should contrib modules reasonable expect the system.theme.css file to be loaded or not?

@quicksketch dealt with a similar problem here by adding properties to Basis: backdrop/backdrop#2070

I see now that @olafgrabienski is suggesting that maybe the changes like this should be made to core/modules/system/css/system.css

Would adding this break existing sites?