Design fixes (#555)

* Change placeholder color and design fixes for show page / sidebar

* Replace hardcoded border radiuses

* Improve border display for middle of button group

* Editor styling

* Editor font size

* Comment Bar positioning and remove scrollbar for 1px

* Add Comments section title

* Nit: match css style

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Co-authored-by: Emilien <emilien.chauvet.enpc@gmail.com>

front/src/modules/ui/layout/top-bar/TopBar.tsx

@@ -31,7 +31,7 @@ const TitleContainer = styled.div`
 const AddButtonContainer = styled.div`
   align-items: center;
   border: 1px solid ${({ theme }) => theme.border.color.medium};
-  border-radius: 4px;
+  border-radius: ${({ theme }) => theme.border.radius.sm};
   color: ${({ theme }) => theme.font.color.tertiary};
   cursor: pointer;
   display: flex;