Fleet UI: Fix dark mode switch flicker on data tables (#47541)

frontend/components/TableContainer/DataTable/_styles.scss

@@ -19,9 +19,9 @@ $shadow-transition-width: 16px;
       /* Shadows */ linear-gradient(
           to right,
           $core-fleet-white,
-          $transparent
+          $core-fleet-white-transparent
         ),
-        linear-gradient(to left, $core-fleet-white, $transparent),
+        linear-gradient(to left, $core-fleet-white, $core-fleet-white-transparent),
         /* Shadow covers */
           linear-gradient(
             to right,

frontend/styles/global/_global.scss

@@ -34,11 +34,14 @@ body {
   body.theme-transition *::after {
     transition: background-color 300ms ease, color 300ms ease,
       border-color 300ms ease, box-shadow 300ms ease, fill 300ms ease,
-      stroke 300ms ease !important;
+      stroke 300ms ease, background-image 300ms ease !important;
   }
 
-  // Hide the gradient image during theme switch so background-color
-  // can transition smoothly without the gradient snapping.
+  // The core-wrapper's hero gradient doesn't interpolate cleanly between
+  // themes (different shape/stops), so hide it during the transition and
+  // let background-color carry the fade. The data-table wrapper keeps its
+  // gradient — it has the same shape in both themes, so background-image
+  // interpolates smoothly along with background-color.
   body.theme-transition .core-wrapper {
     background-image: none !important;
   }
@@ -285,6 +288,26 @@ body.dark-mode tbody tr {
   background-color: $dark-mode-table-row;
 }
 
+// Data table rows stay transparent in dark mode so the wrapper's scroll
+// shadow can show through when the table is horizontally scrollable —
+// matches the light-mode pattern where `tr` has no background and the
+// wrapper shows through.
+body.dark-mode .data-table-block tbody tr {
+  background-color: transparent;
+}
+
+// $ui-off-white-opaque is a near-transparent rgba tuned for light mode;
+// against the dark wrapper it's perceptually invisible. A translucent
+// black overlay darkens the row beneath the wrapper color cleanly — the
+// interpolation from `transparent` (rgba(0,0,0,0)) to rgba(0,0,0,0.2)
+// stays in monotonically darkening tints, so no flicker. Selector
+// includes `.data-table` to beat the component rule's `.single-row:hover`
+// specificity (0,4,2).
+body.dark-mode .data-table-block .data-table tbody tr:hover,
+body.dark-mode .data-table-block .data-table tbody .single-row:hover {
+  background-color: rgba(0, 0, 0, 0.2);
+}
+
 body.dark-mode .site-nav-item:not(.dup-org-logo):hover {
   background-color: $dark-mode-nav-hover;
 }

frontend/styles/var/colors.scss

@@ -12,6 +12,11 @@
   --core-fleet-black: #192147;
   --core-fleet-green: #009a7d;
   --core-fleet-white: #ffffff;
+  // Same RGB as --core-fleet-white with zero alpha. Use as the fade-out
+  // endpoint in gradients that need to disappear into the wrapper background
+  // cleanly. The generic `$transparent` is rgba(255, 255, 255, 0) regardless
+  // of theme, which leaks white into dark-mode gradients.
+  --core-fleet-white-transparent: rgba(255, 255, 255, 0);
   --ui-fleet-black-75: #515774;
   --ui-fleet-black-50: #8b8fa2;
   --ui-fleet-black-33: #b3b6c1;
@@ -118,6 +123,7 @@ body.dark-mode {
   --core-fleet-black: #e2e4ea;
   --core-fleet-green: #00c28b;
   --core-fleet-white: #1a1c21;
+  --core-fleet-white-transparent: rgba(26, 28, 33, 0);
   --ui-fleet-black-75: #bebebf;
   --ui-fleet-black-50: #87888b;
   --ui-fleet-black-33: #636777;
@@ -147,7 +153,7 @@ body.dark-mode {
   --ui-gray: #32363e;
   --ui-light-grey: #1e2128;
   --ui-off-white: #1e2128;
-  --ui-shadow: #000000;
+  --ui-shadow: #2a2e36;
   --ui-vibrant-blue-50: rgba(123, 121, 255, 0.3);
   --ui-vibrant-blue-25: #282736;
   --ui-vibrant-blue-10: #2d2e4d;
@@ -219,6 +225,7 @@ body.dark-mode {
 $core-fleet-black: var(--core-fleet-black);
 $core-fleet-green: var(--core-fleet-green);
 $core-fleet-white: var(--core-fleet-white);
+$core-fleet-white-transparent: var(--core-fleet-white-transparent);
 $ui-fleet-black-75: var(--ui-fleet-black-75);
 $ui-fleet-black-50: var(--ui-fleet-black-50);
 $ui-fleet-black-33: var(--ui-fleet-black-33);