[theme] add border radius to engine token input

searx/static/themes/simple/src/less/preferences.less

@@ -44,6 +44,7 @@
       border: 1px solid var(--color-toolkit-input-text-border);
       background: none repeat scroll 0 0 var(--color-base-background);
       padding: 0.4rem;
+      .rounded-corners-tiny;
 
       &:hover {
         border: 1px solid var(--color-toolkit-input-text-border-hover);