tweak some website layout and style settings

- make previous/next buttoms smaller and show on top and bottom of page
- make tab header items smaller and more rounded
- reduce padding on a few selected items to make pages more compact

remove obsolete lammps.css file variant

doc/utils/sphinx-config/_static/css/lammps.css

@@ -18,3 +18,30 @@
 .versionmodified {
     font-weight: bold;
 }
+
+hr {
+     margin: 12px 0;
+}
+
+.rst-content {
+     margin-bottom: 12px !important;
+}
+
+.ui.tabular.menu .item {
+    padding-right: 1em;
+    padding-left: 1em;
+    padding-top: 0.1em;
+    padding-bottom: 0.1em;
+    border-radius: 6px;
+}
+
+.btn {
+    padding: 4px;
+    font-size: 80%;
+    border-radius: 4px;
+}
+
+.rst-breadcrumbs-buttons {
+    margin: 0;
+}
+

doc/utils/sphinx-config/_themes/lammps_theme/static/css/lammps.css

@@ -1,9 +0,0 @@
-.lammps_version {
-    text-align: center;
-    display: block;
-    margin-bottom: 0.809em;
-}
-
-.versionmodified {
-    font-weight: bold;
-}

doc/utils/sphinx-config/_themes/lammps_theme/theme.conf

@@ -13,6 +13,6 @@ includehidden = True
 titles_only =
 logo_only =
 display_version = True
-prev_next_buttons_location = bottom
+prev_next_buttons_location = both
 style_external_links = False
 style_nav_header_background =