If this works similar to Sigil,
The inline TOC is built from the NCX.
You can't use use any styling in the NCX so it is filtered out as it hits the NCX (no longer there).
Hand edit (it back) time if you want it in the final inline TOC.
remember, this is a fragile step. A rebuild of the TOC wipes your work
You CAN have a custom TOC stylesheet for the overall L&F