Looks like you're using undraw svgs with the default color. You can change the color by clicking the color button in the upper right corner of the undraw site.

I'd add some examples of emails that look good that are done in Markdown. Starter templates would be really helpful.

I'd change the limit from X emails/day to X/month. Who sends mass email every day?

I can't tell what the pricing would be for a much larger number of emails without signing up. Do it the way Mailchimp does -- put your pricing slider outside the signup.