Narrow body with and align gitea icon (#126)

As discussed in the maintainers channel.

## Screenshots

![image](/attachments/64fd58d9-fbbf-4adf-bd56-13d7b2c8f197)

![image](/attachments/d08b4367-9cc5-4faf-b55d-1c3d5940f20a)

![image](/attachments/c3ab77e9-4b12-463e-9846-8a624ba750d2)

Reviewed-on: https://gitea.com/gitea/theme/pulls/126
Reviewed-by: silverwind <silverwind@noreply.gitea.io>
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: pat-s <patrick.schratz@gmail.com>
Co-committed-by: pat-s <patrick.schratz@gmail.com>

src/main.scss

@@ -67,12 +67,13 @@ a.author {
 
 .navbar .navbar-brand {
   align-items: center;
-  justify-content: space-between;
+  justify-content: flex-end;
   flex-grow: 1;
   input {
     height: 50%;
     padding: 0.3rem;
     margin-right: 0.75rem;
+    padding-bottom: 4px;
   }
 }
 
@@ -304,3 +305,18 @@ a:hover {
   border-color: #528321;
   color: #fff;
 }
+
+.content {
+  width: 800px;
+  padding-left: 0px;
+  padding-right: 0px;
+  margin: 0 auto;
+}
+
+.navbar-end {
+  margin-left: inherit;
+}
+
+.page .card {
+  display: table;
+}