Simplify CSS remove z-index (#2861)

* Simplify CSS remove z-index

#fix https://github.com/FreshRSS/FreshRSS/issues/2860
Improvement of https://github.com/FreshRSS/FreshRSS/pull/2845

https://github.com/FreshRSS/FreshRSS/commit/22916d6a3ed259b5a5dca5cddb4a58433729384f#r38143737

* Fix when .nav_menu is not loaded

This ability was lost with the last styling iterations

p/themes/Alternative-Dark/template.css

@@ -869,18 +869,13 @@ br {
 }
 
 /*=== "Load" parts */
-#stream {
-	position: relative;
-	z-index: 5;
-}
 
 /* Only matches while waiting for div#stream to be received */
-#first_load:nth-last-of-type(2) {
+#first_load:nth-last-of-type(1), #first_load:nth-last-of-type(2) {
 	margin: -40px auto 0 auto;
 	height: 40px;
 	position: relative;
-	top: 250px;
-	z-index: 2;
+	top: 290px;
 }
 
 #load_more {

p/themes/Alternative-Dark/template.rtl.css

@@ -869,18 +869,13 @@ br {
 }
 
 /*=== "Load" parts */
-#stream {
-	position: relative;
-	z-index: 5;
-}
 
 /* Only matches while waiting for div#stream to be received */
-#first_load:nth-last-of-type(2) {
+#first_load:nth-last-of-type(1), #first_load:nth-last-of-type(2) {
 	margin: -40px auto 0 auto;
 	height: 40px;
 	position: relative;
-	top: 250px;
-	z-index: 2;
+	top: 290px;
 }
 
 #load_more {

p/themes/base-theme/template.css

@@ -869,18 +869,13 @@ br {
 }
 
 /*=== "Load" parts */
-#stream {
-	position: relative;
-	z-index: 5;
-}
 
 /* Only matches while waiting for div#stream to be received */
-#first_load:nth-last-of-type(2) {
+#first_load:nth-last-of-type(1), #first_load:nth-last-of-type(2) {
 	margin: -40px auto 0 auto;
 	height: 40px;
 	position: relative;
-	top: 250px;
-	z-index: 2;
+	top: 290px;
 }
 
 #load_more {

p/themes/base-theme/template.rtl.css

@@ -869,18 +869,13 @@ br {
 }
 
 /*=== "Load" parts */
-#stream {
-	position: relative;
-	z-index: 5;
-}
 
 /* Only matches while waiting for div#stream to be received */
-#first_load:nth-last-of-type(2) {
+#first_load:nth-last-of-type(1), #first_load:nth-last-of-type(2) {
 	margin: -40px auto 0 auto;
 	height: 40px;
 	position: relative;
-	top: 250px;
-	z-index: 2;
+	top: 290px;
 }
 
 #load_more {