Update Dropdown type & add accent color to theme

[emplums]

This PR fixes a few issues brought up by another GitHub team:

• Dropdown title type definition is clashing with the CommonProps types that it extends. It was also on DropdownMenu when it should have just been on Dropdown.

  BaseTypes extends React.Props<any> and adds the title attribute as an acceptable type. I'm honestly not sure why title is included in BaseTypes 🤔 but because it's set as only a string it conflicts with the title prop in Dropdown. I ended up needing to redefine all the types we want from BaseTypes and CommonTypes on Dropdown to get around this.

• Changed the theme call in UnderlineNav to access colors.accent instead of colors.orange[5] to allow users of the component to override this color if needed by changing the accent color in the theme

[vercel]

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