silverstripe-framework/admin/client/src/components/form-action/styles.scss

// General buttons
.btn {
	height: 32px;
	margin-right: 1rem;
	position: relative;
}

// Button icons
.btn[class*="font-icon-"]::before {
	font-size: 16px;
	position: relative;
	top: 3px;
	margin-right: 6px;
	line-height: 13px;
}

.no-text[class*="font-icon-"]::before {
	margin-right: 0;
}

.btn-group {
	margin-right: 1rem;

	.btn {
		margin-right: 0;
	}

	.btn-success {
		border-left: 1px solid darken($btn-success-bg, 6%);

		&:first-child {
			border-left: none;
		}
	}
}

// SVG loading icon
.btn__loading-icon {
	float: left;
	margin: 0 4px 0 0;
	height: 20px;
	position: absolute;
	left: 50%;
	top: $btn-padding-y;
	transform: translate(-50%);

	svg {
		width: 24px;
		height: 20px;

		circle {
			width: 4px;
			height: 5px;
			animation: loading-icon 1.2s infinite ease-in-out both;
			fill: $gray;
			transform-origin: 50% 50%;
		}
		
		circle:nth-child(1) {
			animation-delay: -.32s;
		}
		
		circle:nth-child(2) {
			animation-delay: -.16s;
		}
	}
}

.btn--loading {
	> span,
	&::before {
		visibility: hidden;
	}
}

@keyframes loading-icon {
	0%, 80%, 100% { transform: scale(0); }
	40% { transform: scale(1); }
}

// Specific button types
.btn-link {
	&:hover,
	&:focus {
		text-decoration: none;
	}
}

.btn-secondary {
	border-color: transparent;

	&:hover,
	&:active,
	&:active:focus,
	&:active:hover,
	&:focus {
		background-color: $gray-lighter;
	}
}

.btn-success-outline {
	border-color: lighten($brand-success,10%);

	&:hover,
	&:active,
	&:focus {
		color: $brand-success;
		background-image: none;
		background-color: transparent;
		border-color: lighten($brand-success,10%);
	}

	svg circle {
		fill: $brand-success;
	}
}

.btn-success {
	box-shadow: 0 1px 0 $btn-success-shadow;

	svg circle {
		fill: #fff;
	}
}