Replace box-shadow for floating dropdown as well (#26581)

Add `box-shadow` replacement to the `floating` dropdown variant as well,
which was missed in #26469. The
Fomantic style has `!important`, so this has to have too. Also made a
tiny adjustment to shadow color on dark theme.

<img width="305" alt="Screenshot 2023-08-18 at 16 40 34"
src="https://github.com/go-gitea/gitea/assets/115237/a0aac9cb-6393-4d69-b0b3-00eaac5ccf9f">
<img width="202" alt="Screenshot 2023-08-18 at 16 40 22"
src="https://github.com/go-gitea/gitea/assets/115237/0a5fa3aa-7452-4dbd-86ed-ccbc1c872ebb">

Co-authored-by: Giteabot <teabot@gitea.io>

web_src/css/base.css

@@ -1204,6 +1204,7 @@ img.ui.avatar,
   box-shadow: none;
 }
 
+/* replace fomantic popover box shadows */
 .ui.dropdown .menu,
 .ui.upward.dropdown > .menu,
 .ui.menu .dropdown.item .menu,
@@ -1213,6 +1214,9 @@ img.ui.avatar,
 .ui.upward.active.selection.dropdown:hover .menu {
   box-shadow: 0 6px 18px var(--color-shadow);
 }
+.ui.floating.dropdown .menu {
+  box-shadow: 0 6px 18px var(--color-shadow) !important;
+}
 
 .ui.dimmer {
   background: var(--color-overlay-backdrop);

web_src/css/themes/theme-arc-green.css

@@ -211,7 +211,7 @@
   --color-button: #1e252e;
   --color-code-bg: #222733;
   --color-code-sidebar-bg: #232834;
-  --color-shadow: #00000060;
+  --color-shadow: #00000058;
   --color-secondary-bg: #2a2e3a;
   --color-text-focus: #fff;
   --color-expand-button: #3c404d;