Update scroll-margin-top (#8641)

## Describe your changes

The app header bar height changed in
#8554 and now the heading can
be overlayed by it when clicking on the anchor. This PR fixes this by
using the correct height.

## GitHub Issue Link (if applicable)

## Testing Plan

- manual tests

---

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project are made under the Apache 2.0 license.

frontend/lib/src/components/shared/StreamlitMarkdown/styled-components.ts

@@ -154,7 +154,7 @@ export const StyledLinkIcon = styled.a(({ theme }) => ({
 
 export const StyledHeadingWithActionElements = styled.div(({ theme }) => ({
   "h1, h2, h3, h4, h5, h6, span": {
-    scrollMarginTop: theme.spacing.threeXL,
+    scrollMarginTop: theme.sizes.headerHeight,
   },
   ...sharedMarkdownStyle(theme),