Currently we've OOTB styles plugin configured for RTE in AEM 6.3, we need to add second styles plugin(duplicate styles plugin with name "sizes").
I tried to use the same script from styles which i found under the path
but not working fine. Tried to use the same functions from StylesPluginImpl.js too.
I too don't see the check mark
Regarding the styles they act toggle so we need to uncheck the existing one before we apply a new style if you don't want to see the old style in source code