move css into separate files

_includes/header.css

@@ -0,0 +1,57 @@
+header {
+  display: flex;
+  position: sticky;
+  top: 0;
+  min-height: 6.4rem;
+  padding: 2rem 0;
+  backdrop-filter: blur(5px);
+  background-color: rgba(255,255,255,.92);
+  box-shadow: 0 3px 15px hsla(0,0%,39.2%,.12);
+}
+
+.header-container {
+  width: 100%;
+  max-width: var(--page-width);
+  margin-left: auto;
+  margin-right: auto;
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+}
+
+#mainNav ul {
+  margin: 0;
+  padding: 0;
+}
+
+#mainNav li {
+  display: inline;
+  margin-left: 1rem;
+}
+
+#mainNav a {
+  position: relative;
+  font-size: 16px;
+  font-family: Aileron;
+  font-weight: 500;
+  line-height: 2;
+  text-transform: uppercase;
+  color: var(--color-text);
+  text-decoration: none;
+}
+
+#mainNav a::after {
+  content: "";
+  position: absolute;
+  background: var(--color-primary);
+  opacity: 0;
+  bottom: -4px;
+  left: 0;
+  right: 0;
+  height: 2px;
+  transition: opacity .3s ease-out;
+}
+
+#mainNav a:hover::after {
+  opacity: 1;
+}