Improve color of the Mathesar link in forms

[pavish]

Description

• The Mathesar link color on the form fillout page appears visually out of place compared to the rest of the theme.
• Originally reported by @seancolsen in Standardize colors #4563
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What needs to be done

• Update the link color to use the base text color from our theme.
• Add an underline during hover/focus.
• Verify that it looks good in both light and dark themes.
• Whoever picks this issue should ensure that the result looks visually aligned and use their own judgement. Further discussions on color preferences can happen in the PR review threads.

[pavish]

I've split #4901 away from this issue and kept this focused on the Mathesar link color in forms.