luci-theme-openwrt: dropdown style tweaks

- Remove 2em min height, this looks out of place with the OpenWrt theme
- Only colorize invalid dropdown when not open
- Drop use of image background for invalid fields while we're at it

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

themes/luci-theme-openwrt/htdocs/luci-static/openwrt.org/cascade.css

@@ -620,10 +620,7 @@ img.cbi-image-button {
 	color: #595;
 }
 
-.cbi-input-invalid {
-	background-image: url('../resources/cbi/reset.gif');
-	background-repeat: no-repeat;
-	background-position: right;
+.cbi-input-invalid:not([open]) {
 	color: #FF0000 !important;
 	border-color: #FF0000;
 }
@@ -1044,7 +1041,6 @@ ul.cbi-tabmenu li.cbi-tab {
 	flex-direction: column;
 	justify-content: center;
 	text-align: center;
-	line-height: 2em;
 	padding: 0 .25em;
 }