At present the only way to customise the buttons is by adding custom classes to the [ddownload] shortcode.
You will first need to add the CSS classes to your themes file. I'd advise against adding them to the Delightful Downloads file as you will lose the changes when you update the plugin.
To use the new button class you just add them to the shortcode like so:
[ddownload id="1" class="my_custom_button"]
Please be aware that these new defined styles will not show up in the colour options and in order for you custom styles to work you may need to add !important to the properties. Also, if you do not want to use any of the provided buttons you can disable the CSS file from being included in the Delightful Downloads Settings screen.
Although not an ideal solution it is a workaround and this is definitely something I want to improve in a future update.