UI: add dark theme

[Aman-Codes]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Added dark theme
Closes #3775

Screenshots

Homepage

Status page

Store page

Verification

cd pkg/ui/react-app/
yarn start

[bwplotka]

OMG we need this.

Thanks! It looks good on outside, but we need @onprem blessing here 🙈

[roidelapluie]

This pull request seems to copy/paste code from Prometheus without the attribution required by the Apache-2 License.

[Aman-Codes]

This pull request seems to copy/paste code from Prometheus without the attribution required by the Apache-2 License.

@roidelapluie To maintain consistency in Thanos and Prometheus UI, I reused the Prometheus dark theme code.
I have now added the License attribution. Let me know if anything more is required.

[onprem]

LGTM, Thanks!

[kakkoyun]

LGTM 🚀

Noob question: Is there a way to directly import the theme from Prometheus rather than copying over the code? It'll be hard to keep it in sync?

[onprem]

Is there a way to directly import the theme from Prometheus rather than copying over the code? It'll be hard to keep it in sync?

Currently, no. We have to publish relevant bits on npm to do that.

[Aman-Codes]

I only fixed the merged conflict. The dismissed stale reviews is due to saacs/github#1157