Closes #922 - Dark mode fixes

app/components/Collaborators.js

@@ -145,7 +145,7 @@ const More = styled.div`
   border-radius: 12px;
   background: ${props => props.theme.slate};
   color: ${props => props.theme.text};
-  border: 2px solid #fff;
+  border: 2px solid ${props => props.theme.background};
   text-align: center;
   line-height: 20px;
   font-size: 11px;

shared/styles/theme.js

@@ -138,7 +138,7 @@ export const dark = {
   codeBackground: colors.almostBlack,
   codeBorder: colors.black50,
   embedBorder: colors.black50,
-  horizontalRule: colors.almostWhite,
+  horizontalRule: darken(0.2, colors.slate),
 };
 
 export default light;