From OpenSCADAWiki
In general, the styles are available from the project level. At the level of the widget libraries you can only define the style fields for the widgets. At the project level, at the choice of style, it is started the work with styles, which includes access to the fields of styles instead of direct attribute values. In fact, this means that when reading or writing a widget attribute these operations will be carried out with the corresponding field of the chosen style. The configuration field "Write to style in the execution context" allows of dynamic changing the style fields in the execution context by writing to the proper attributes, but that can be done in wrong, so that is why the field is disabled by default.