Change all <input> buttons to <button type="submit">
Sensei buttons are the worst part about theming Sensei. This is because there markup is hardcoded into the Sensei classes and they are defined at input buttons.
You can add CSS overrides to style the color, border and padding,,, but that is about it as the type of <input> does not support most of the CSS standard. If for example you wanted to add a font based icon to the button this is not possible.
Typically you would use the CSS:before property to append the icon to the element, but the CSS:before property is not supported on an input tag. We could use text filters to add additional markup to the button, but as an <input> the html is rendered at text and not html.
This means that the only way to have flexible styling for the buttons is to modify the Sensei classes. Something you should never do.