(core) Adjust the style of the 'attic' to have more of a toolbar feel, flush to the top, and taking up less space

Test Plan: Checked new looks manually, behavior should not be affected.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3934

app/client/components/buildViewSectionDom.ts

@@ -195,7 +195,6 @@ const cssResizing = styled('div', `
 const cssMiniSection = styled('div.mini_section_container', `
   --icon-color: ${colors.lightGreen};
   display: flex;
-  background: ${theme.mainPanelBg};
   align-items: center;
   padding-right: 8px;
 `);