@CaitlynMohr
This lesson in the Canvas Administrator's Guide explains how inheritance works with themes: How do I create a theme for an account using the Theme Editor?
In short, items not specifically set in the subaccount theme inherit the values from the account theme. Values changed in a subaccount override those in the account.
The account theme will override a subaccount if you have not specified that setting in the subaccount theme. That's the notion of inheritance -- values not specifically set are inherited from the parent theme.
To edit a sub-account theme, you go to Admin > Subaccounts and choose the subaccount. Then click Themes. If you just go to Admin > Themes, you're changing it for the entire account. You can look at the breadcrumb to verify which [sub]account you're working with.