I was able to change the button colour, button border, button effects on hover.
To do this first create a directory in assets/css and in root directory create a file style.scss. In that file paste the following code

---
---
@import "{{ site.theme }}";
.btn {

color: #0000ff;

border-color: #0000ff;

// background-color: #008CBA;

}
.btn:hover {

color: #0bb6e1cc;

text-decoration: none;

background-color: #7c940ea1;

border-color: #0000ff;

}