DOC: style main page card. (numpy#21456)

[skip actions] [skip travis]

Author: tupui

doc/source/_static/numpy.css

@@ -22,7 +22,6 @@ h1 {
   color: #013243; /* warm black */
 }
 
-
 h2 {
   color: #4d77cf; /* han blue */
   letter-spacing: -.03em;
@@ -32,3 +31,24 @@ h3 {
   color: #013243; /* warm black */
   letter-spacing: -.03em;
 }
+
+.intro-card {
+  padding: 20px 10px 20px 10px;
+  margin: 10px;
+}
+
+.intro-card p.card-text {
+  margin: 0;
+}
+
+.intro-card .card-img-top {
+  height: 52px;
+  width: 52px;
+  margin-left: auto;
+  margin-right: auto;
+}
+
+.intro-card .card-header {
+  margin-top: 20px;
+  font-size: var(--pst-font-size-h5);
+}