ui: Reduce the gap at the top of the pages

- So much spacing is not required as we have remove the background and borders.

- This is similar in approach to Bootstrap 4/5 which does not provide top
margins for heading elements and instead provides them only when they are not
the first element.

- Increase the margin between the end of a section and beginning of the next
heading.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Veiko Aasa <veiko17@disroot.org>

static/themes/default/css/main.css

@@ -73,10 +73,13 @@
 /*
  * Bootstrap override
  */
-/* Heading margins from bootstrap 3 */
-h1, h2, h3, h4, h5, h6 {
-    margin-top: 1.25rem;
-    margin-bottom: 0.625rem;
+h1:not(:first-child),
+h2:not(:first-child),
+h3:not(:first-child),
+h4:not(:first-child),
+h5:not(:first-child),
+h6:not(:first-child) {
+    margin-top: 2rem;
 }
 
 /* Warning colors from bootstrap 3 */
@@ -813,7 +816,7 @@ input[type='submit'].running-status-button {
 }
 
 .app-header > img {
-    margin: 1.5625rem auto 0;
+    margin: 0 auto;
     width: 100%;
 }