Style polish for top nav redesign

What does this MR do?

This MR addresses the UX polish feedback over in this comment:

• The padding between icon and text is only 4px, I think we usually have 8px for these cases.
• Border should be using #F0F0F0 $gray-50 instead of #bfbfbf
• Hover state of the frequently visited items should match other menu hover state. The frequently visited items is missing border radius.

• The 12px x 8px button padding will not be changed because it's part of the component's standard padding !63241 (comment 592330599)
• We will change the color of the frequent items item titles in a follow-up

Screenshots (strongly suggested)

Description	Before	After
With Feature.disable(:combined_menu), notice that we technically change the border radius of the buttons
With feature enabled, notice that the border radius of the frequent items has changed.
With feature enabled on mobile, notice the border radius
With feature enabled, notice the separator color
With feature enabled, notice the spacing between icon and text

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)