The Manage style dialog allows several things:
The existing styles are listed in the high part of the dialog. They appear with an icon which has various meanings:
for a standard style
for a standard style you modified or for a custom style

According to the type of the selected style, you can execute various actions:
Standard style Rename
Standard style you modified Rename, Reset to default
Custom style Rename, Remove

The followed actions are available:
Rename Allows to rename the selected style.
Reset to default Allows to delete the customizations of a standard style and thus to return it to the default values for this style. It concerns as well the name of the style as the associated settings.
Remove Allows to remove the personalized style.