[MM-29842] Migrate changeCSS() to CSS variable in utils/utils.jsx

[1jz]

Summary

Removes the call to changeCss() for the disabled send button mentioned in the issue below and adjusts the buttons color to --center-channel-color to the scss file.

Ticket Link

Fixes mattermost/mattermost#16028

[mattermod]

Hello @1jz,

Thanks for your pull request! A Core Committer will review your pull request soon. For code contributions, you can learn more about the review process here.

Per the Mattermost Contribution Guide, we need to add you to the list of approved contributors for the Mattermost project.

Please help complete the Mattermost contribution license agreement?
Once you have signed the CLA, please comment with /check-cla and confirm that the CLA check is green.

This is a standard procedure for many open source projects.

Please let us know if you have any questions.

We are very happy to have you join our growing community! If you're not yet a member, please consider joining our Contributors community channel to meet other contributors and discuss new opportunities with the core team.

[hanzei]

Hey @1jz, thanks for opening a PR! Would you be open on signing the contribution license agreement?

[1jz]

/check-cla

[Willyfrog]

LGTM! thanks for contributing

[nevyangelova]

LGTM! - 1 Minor conflict to resolve :)

[1jz]

Conflict should be resolved, thank you for your patience :)

[1jz]

I didn't pay close attention and I ended up re-introducing one of the migrated calls in the last merge, it should be removed now.