Bootstrap includes six predefined button styles, each serving its own semantic purpose.

Use a class .btn-outline-{color}to quickly create a outline button.

Use .btn-flat-{color}to create a flat button

use bg-gradient-{color}create gradient buttons

Use .btn-relief-{color}to create a relief button

Use a class .squarewith outline button class to create square outline button.

Use a class .roundwith outline button class to create round outline button.

You can change the font color of buttons, using .text-{color}