Top panel drop shadow (#1432)

closes https://github.com/webstudio-is/webstudio-builder/issues/1419

## Description

1. What is this PR about (link the issue and add a short description)

## Steps for reproduction

1. style panel shadow on the top panel
 

## Code Review

- [ ] hi @kof, I need you to do
  - conceptual review (architecture, feature-correctness)
  - detailed review (read every line)
  - test it on preview

## 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-builder/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/style-panel.tsx

@@ -90,7 +90,7 @@ export const StylePanel = ({ selectedInstance, publish }: StylePanelProps) => {
         css={{
           px: theme.spacing[9],
           pb: theme.spacing[9],
-          boxShadow: `0px 1px 0 ${theme.colors.panelOutline}`,
+          boxShadow: theme.shadows.panelSectionDropShadow,
         }}
       >
         <StyleSourcesSection />