You can’t style them as the like button is an iframe and the reblog button is a link with an SVG embed. Theoretically, you could style the reblog button by using :before in your CSS (I already do this in my own themes for other icons, you can take a look at my coding), but the like button cannot be styled at all besides the options Tumblr gives you (size, white/black/grey).
Slightly off-topic - I’ve noticed over the past few years that the web design trends are headed towards flatter designs with larger font sizes and larger elements in general. Tumblr’s new like and reblog buttons reflect this change as they look much better at larger sizes. This doesn’t work well with a lot of Tumblr themes that have a very specific kind of aesthetic (themes that are based on movies, TV shows, books, etc, or have patterned backgrounds or textures or lots of small details), but they do work well with clean, minimalist themes with large font sizes and elements. Keep that in mind if you choose to use them.