The values can be:
  "requested" - the style as requested
  "regular"   - use a regular full-color icon
  "symbolic"  - use a symbolic icon

The property defaults to "requested", so no changes should be seen
unless CSS overrides it.
It is also inherited, so that using this CSS
  .toolbar { -gtk-icon-style: symbolic; }
is enough to force the whole toolbar to use symbolic icons.