Use css variables.

styles/index.css

@@ -1,7 +1,7 @@
 html,
 body {
   font-family: Inter, -apple-system, BlinkMacSystemFont, sans-serif;
-  font-size: 16px;
+  font-size: var(--font-size-normal);
   font-weight: 400;
   line-height: 1.8;
   padding: 0;
@@ -9,8 +9,8 @@ body {
   width: 100%;
   height: 100%;
   box-sizing: border-box;
-  color: #2c2c2c;
-  background: #fafafa;
+  color: var(--gray900);
+  background: var(--gray75);
 }
 
 *,
@@ -19,6 +19,10 @@ body {
   box-sizing: inherit;
 }
 
+h2 {
+  font-weight: 400;
+}
+
 #__next {
   display: flex;
   flex-direction: column;
@@ -35,12 +39,7 @@ select {
 a,
 a:active,
 a:visited {
-  color: #2680eb;
-}
-
-header a {
-  color: #2c2c2c !important;
-  text-decoration: none;
+  color: var(--primary400);
 }
 
 form label {
@@ -48,13 +47,19 @@ form label {
   min-width: 100px;
 }
 
-form input {
+form input,
+form textarea {
+  padding: 4px 8px;
   margin-right: 10px;
+  margin-bottom: 20px;
+  border: 1px solid var(--gray500);
+  border-radius: 4px;
+  outline: none;
 }
 
 select {
   padding: 4px 8px;
-  border: 1px solid #b3b3b3;
+  border: 1px solid var(--gray500);
   border-radius: 4px;
 }