Remember, Ionic is just HTML,CSS, and JS. If you have particular styles that you want to incorporate into ionic, then you can write some css and create your own classes. Think customizing bootstrap. If you have a build process, you can incorporate ionic’s scss files and go from there.
If you’re creating project using the ionic CLI, you should have scss file already included. There you can write your own styles and theme then run gulp sass or gulp watch to compile the scss into css
Frankly, I wish there was an Android theme - Ionic apps look like iOS 7 apps on an Android device. Android needs some love too. Ideally there would be a per-platform option to swap out themes when an app is built.
Something that we are actually working on.
Less of theme-appearance thing, but changing things like animations, tabs appearance, nav-bar title align, etc