You can remove the border by adding border-0 to <a class="btn btn-outline-secondary text-center rounded-0 m-2"

As for custom colours when hovering over buttons: You can only apply this with CSS.

You can however use Bootstrap colours. You'll need to change btn-outline-secondary to btn-outline-primary for blue...

btn-outline-danger for red | btn-outline-success = green | btn-outline-warning = yellow | btn-outline-info = light blue

More info on bootstrap colours here: https://getbootstrap.com/docs/4.0/utilities/colors/