In bug 101648 the functions New and Modify have been disabled (actually both are hidden) but ultimately we need a dialog. The design team made a proposal which is published az https://design.blog.documentfoundation.org/2015/12/13/style-your-tables/
Confirming of course.
Best regards. JBF
As a workaround you can use the right-most tool button in the styles deck to either modify the selected table style or to create a new. However, those changes are stored only in the current document.
IMHO this bug should have a higher importance because table styles without the possibility to create or modify styles are useless in a large part. Also the table style feature was prominently promoted in the release notes.