fix: apply borderless view in dark mode (#3716)

## 📝 Summary

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#123).
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Borderless mode isn't applied in dark mode, this is to fix that
<img width="793" alt="image"
src="https://github.com/user-attachments/assets/2e0cedeb-9782-455d-9923-249999f3ece2"
/>

## 🔍 Description of Changes

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- [X] I have read the [contributor
guidelines](https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md).
- [ ] For large changes, or changes that affect the public API: this
change was discussed or approved through an issue, on
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- [ ] I have added tests for the changes made.
- [X] I have run the code and verified that it works as expected.

## 📜 Reviewers

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@akshayka OR @mscolnick

frontend/src/css/app/Cell.css

@@ -202,14 +202,7 @@
 
   /* Borderless styles for Cell */
   &.borderless {
-    border-color: transparent;
-    box-shadow: none;
-
-    /* Apply the original styles */
-    &:hover {
-      border: 1px solid var(--gray-4);
-      @apply shadow-smSolid shadow-shade;
-    }
+    @apply borderless-common;
   }
 
   /* -------------------------- Shoulders/Buttons ---------------------------- */
@@ -233,8 +226,23 @@
   @apply bg-background;
 }
 
+.borderless-common {
+  border-color: transparent;
+  box-shadow: none;
+
+  /* Apply the original styles */
+  &:hover {
+    border: 1px solid var(--gray-4);
+    @apply shadow-smSolid shadow-shade;
+  }
+}
+
 .dark .Cell {
   @apply border-border;
+
+  &.borderless {
+    @apply borderless-common;
+  }
 }
 
 #App.disconnected {