fix: Remaining styling fixes for box shadow and backgrounds (#2818)

Closes https://github.com/webstudio-is/webstudio/issues/1889

## Description

Small  styling fixes 

## Steps for reproduction

1. click button
2. expect xyz

## Code Review

- [ ] hi @TrySound , I need you to do
  - conceptual review (architecture, feature-correctness)

## Before requesting a review

- [ ] made a self-review
- [ ] added inline comments where things may be not obvious (the "why",
not "what")

## Before merging

- [ ] tested locally and on preview environment (preview dev login:
5de6)
- [ ] updated [test
cases](https://github.com/webstudio-is/webstudio/blob/main/apps/builder/docs/test-cases.md)
document
- [ ] added tests
- [ ] if any new env variables are added, added them to `.env.example`
and the `builder/env-check.js` if mandatory

apps/builder/app/builder/features/style-panel/sections/backgrounds/background-thumbnail.tsx

@@ -15,6 +15,7 @@ import { toPascalCase } from "../../shared/keyword-utils";
 
 const Thumbnail = styled("div", {
   borderRadius: 2,
+  borderWidth: 0,
   width: theme.spacing[10],
   height: theme.spacing[10],
 });

apps/builder/app/builder/features/style-panel/sections/backgrounds/backgrounds.tsx

@@ -230,7 +230,7 @@ export const BackgroundsSection = (props: RenderCategoryProps) => {
     >
       <Flex gap={1} direction="column">
         <CssValueListArrowFocus dragItemId={dragItemId}>
-          <Flex gap={1} direction="column" ref={sortableRefCallback}>
+          <Flex direction="column" ref={sortableRefCallback}>
             {sortableItems.map((layer, index) => (
               <Layer
                 id={layer.id}

apps/builder/app/builder/features/style-panel/sections/box-shadows/box-shadow-layer.tsx

@@ -95,7 +95,7 @@ export const Layer = (props: LayerProps) => {
             />
           </>
         }
-      ></CssValueListItem>
+      />
     </FloatingPanel>
   );
 };

apps/builder/app/builder/features/style-panel/shared/color-thumb.tsx

@@ -38,7 +38,7 @@ const style = css({
   height: theme.spacing[10],
   backgroundBlendMode: "difference",
   borderRadius: 2,
-  borderStyle: "solid",
+  borderWidth: 0,
 });
 
 export const ColorThumb = forwardRef<