You can change your email template's default colors:
- Find an email blast that used/is using the Mega-Theme email template. IMPORTANT: Make sure the Theme → Current custom theme is your Mega-Theme email template. If it's not, see here.
- Go to it's Theme → Current custom theme → HTML template.
- At the top of the template, you'll see all the colors the theme uses as variables. You can edit their values by changing their hex codes (i.e. #023d5d). NOTE: The button background colors inherit the values of footer_bg_color and border_color.