[UI] Add new themes for Alert component

[martinrrm]

Adding the new themes to the alert component, this PR only changes CSS files, actual usage of the component will be in future PR's.

Light theme

Danger

Warning

Success

Info

Dark theme

Danger

Warning

Success

Info

Addresses https://github.com/NuGet/NuGetGallery/issues/9862

[Nirmal4G]

@martinrrm Warning should be yellow (bright or dim depending on the base theme) and not orange-ish! It'll be difficult to distinguish error and warning at a glance. When I glanced at this page, I thought both colors were the same. And no, I do not have any issues with recognising colors.

Also, Info can be light/dark blue near to the NuGet accent color instead of Grey-ish!

[martinrrm]

@Nirmal4G Hi! Thanks for your feedback, right now we are trying to stick with Fluent UI color tokens, nevertheless I agree with you, specially in the Light Theme is hard to distinguish the difference. This is another option we can use.

Also, I loved your idea for the info to be Info color, let me double check that with the designers/team

This is a possible alternative for the colors.

[Nirmal4G]

How about this one for the warning? GitHub uses it.

Dark Theme Warning Color	Light Theme Warning Color

These are the default colors on GitHub but you could adjust the hue (brightness/contrast) to make it work on NuGet Gallery.

[martinrrm]

@Nirmal4G Let me check with designers, I don't think we should be using colors from other products given that there is a well-documented color theme for the components we are using.

[Nirmal4G]

@martinrrm Yeah. That's just for visual reference. Adjust the color to match NuGet branding. If you have existing color code, It would not be far off, IMO.