Are you using the dbc.css stylesheet from the Theme Explorer?
It works by using named Bootstrap theme colors, so there is no need for a callback when the theme changes.
To make subtle changes like you are describing is a little trickier - it’s hard to come up with something that works for both (or all) themes. This post might be helpful. How to make ThemeSwitchAIO change the css class? - #4 by AnnMarieW
Another option is to create a new class something like “dbc-light” and “dbc-dark” to customize things for the light and dark themes. However this would have to be updated in a callback when the theme changes.
It might be possible to create it so that this class can go on the outer container of the app, so it only has to be added in one place.