All code are beautifully written. You can customize easily. Just look the amex/ui-button.html file go to form code with change and use.you would like to edit a specific section of the site, simply find the appropriate label in the and change what you need.If you change the "Button" find the following section in the amex/scss/pages/ui-button.scss file
amex bootstrap 4 admin dashboard provide this beautifull with standard button form you can see this documentation
Basic button style
The .btn classes are designed to be used with <button>, <a> and <input> elements.
Square button
Add .no-radious used with <button>, <a> and <input> elements use this button
Rounded button style
Add .btn-rounded and btn-primary btn class use this rounded button
Rounded with outline hover button
Add .btn-rounded and btn-outline-primary btn class use this button
Default border with color transition
Add .btn-primary-hover to use hover button.
No border with color transition
Add .no-border and .btn-primary-hover to use button.
Basic style with hover button
Add .btn-hover-shutter choose your button.
Active button
Add .active choose your button.
Disabled button
Add class .disabled use this button
Cheeck box button
Add class .disabled use this button
Button group
Add button class use this button type
Button tags
Add button class use this button type
LinkRadio button
Add button class use this button type
Icon button
Use any icon use this button
Block level
Add class .btn-block use this button
Button sizing
Add class .btn-lg use this button
Add class .btn-xs use this button
Add class .btn-sm use this button