Making the accounts-ui login buttons look nicer

Is their an easy way to tweak the appearance of the login buttons?

Preferably without going to the hassle of completely customizing the login form.
Perhaps just some small css tweaks? Changing the colour of the button, making the text boxes full screen with etc.

Sure. Just inspect the elements to find their classes / IDs and override them in your own CSS. So long as your CSS us included after and has a higher specifity it will use your own styles.