Hi @JadynTHONE ...
Based on this Guide, How do I manage themes for an account?, it is my understanding that sub-account themes do not carry over into the main root account. However, the opposite is true. If you had customized a theme at the main root account, it would carry over into any of your sub-accounts. But, if you then made some tweaks to a sub-account theme, those changes would only apply to that sub-account (and any sub-accounts that might be below it). Taking a look at the last sentence of the second paragraph in the light blue box of the Guide I linked to above, it says, "When the Theme Editor is enabled for sub-accounts, any design elements they do not specifically change will inherit the account level design."
I hope this is helpful for you. Sing out if you have any questions about this. Thanks!