Resolve "fix responsive style of header to mobile"

Closes #38 (closed)

src/css/custom.css

@@ -76,6 +76,7 @@ p {
   z-index: 1; 
  } 
 
+/************ Header style ***********/
 header {
   height: 500px;
   clip-path: polygon(
@@ -84,6 +85,13 @@ header {
     100% calc(100% - 6vw),0 100%)
 }
 
+.hero__subtitle {
+  width: 90%;
+  text-align: center;
+  margin: auto;
+  padding-bottom: 20px;
+}
+
 /************ main elements style***********/
 .section-promises {
   padding: 4rem 0;