You can add a custom CSS file to the account area by creating a file named account-area.css in your public directory.
Example contents where we're overriding the default colours:
:root {
--bpa-color-primary: deepskyblue;
--bpa-color-success: forestgreen;
--bpa-color-warning: darkorange;
--bpa-color-error: red;
--bpa-color-helpbox: aliceblue;
--bpa-color-content-background: ghostwhite;
}