Add .hidden and .progress-bar

assets/sass/layout.scss

@@ -573,3 +573,34 @@ button, .button {
 		}
 	}
 }
+
+.hidden {
+	display: none;
+}
+
+.progress-bar {
+	position: relative;
+	display: block;
+	margin: 8px;
+	padding: 4px;
+	background: #fff1;
+	border-radius: 5px;
+	overflow: hidden;
+	text-align: center;
+
+	.content {
+		position: relative;
+	}
+
+	&::before {
+		content: "";
+		display: block;
+		position: absolute;
+		left: 0;
+		top: 0;
+		width: var(--progress);
+		height: 100%;
+
+		background: $secondary;
+	}
+}