Child theme for different CSS styles

Hi All,

I would like to ask if I can create a child theme now, despite the fact I have already built multiple sites from the theme.

My client would like to have different CSS mod's on certain sites as part of their branding, (long story, some <h2> headings need to be orange, some are pink etc). So I think I need two child themes, ie, Theme A and Theme B and apply that theme to the relevant site. Am I right or completely off target? Appreciate any help on this one.

Many thanks!