Move submit button to right of email form

I have recently tweaked my CSS on this plugin and I do feel that some CSS selectors and classes could be made better for advanced users.

BUT, try this for your button:

.mc_signup_submit input[type="submit"] {
float: right;
}

I did something similar, where I floated the submit button to the right and the mc_merge_var class to the left. That gave me something that looks like this:

Screenshot — https://cl.ly/image/3h30093A0w41

Hope that helps!

Actually, in mailchimp.php, find

.mc_signup_submit {
text-align:center;
}

change it to

.mc_signup_submit {
text-align:right;
}

Remember that editing a plugin or theme directly that you will loose changes upon an update.
WordPress recommends creating a child theme, it is safest to make changes there.
https://codex.www.ads-software.com/Child_Themes
ADDED:
You can also download a custom CSS plugin and add changes there.