M': Can't change the style of variables, units, etc

I converted a simple document from M15. Came across OK. But the I can't apply/change the style to match my M15 documents.

Help says:

To Apply a Label
1. Select the element that you want to label.
2. On the Math tab, in the Style group, click Labels.
3. Select one of the following labels:
* Variable
* Unit
* Constant
* Function
* System
* Keyword

The selected label is applied to the element.

But it is not. In fact, my styles are greyed out, so I don't think that they are active.

I've gone to Formatting>Label Styles and changed some of the styles. That seems to work.

~R~