Fix terms page in dark mode

What does this MR do and why?

1. Change the rectangular fade at the bottom to adapt to dark mode. Simplified the linear-gradient by swapping the two colors and removing the unneeded degrees and percentages. I changed the transparency so it starts from 100% transparent on the top.

2. Increased the padding around the buttons on the bottom.

Closes #388949 (closed)

Screenshots or screen recordings

Before

A. Dark mode

B. Light mode

After

A. Dark mode

B. Light mode

How to set up and validate locally

Enforce the terms and conditions:

1. In the Admin area > Settings > General > Terms and conditions, add your terms: https://gdk.test:3443/admin/application_settings/general#js-terms-settings
2. Select All users must accept the Terms of Service and Privacy Policy to access GitLab checkbox
3. Select Save changes

Then:

1. Go to preferences and change the color theme to Dark mode (alpha)
2. Go to the https://gdk.test:3443/-/users/terms

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.