[JENKINS-71479] Do not use SCSS lighten function directly to avoid invalid CSS

[dwnusbaum]

See JENKINS-71479.

#7850 in 2.402 migrated frontend styles from LESS to SCSS, but accidentally broke the generated CSS styles for --btn-primary-bg-hover and --btn-primary-bg-active. Before that PR, the generated styles in styles.css were as follows:

--btn-primary-bg:#063f61;
--btn-primary-bg-hover:#085685;
--btn-primary-bg-active:#0a649b;

After that PR, the generated styles are these:

--btn-primary-bg:#063f61;
--btn-primary-bg-hover:lighten(#063f61,7.5%);
--btn-primary-bg-active:lighten(#063f61,12%);

As far as I understand, the latter is invalid CSS, and in my case it seems to be treated as transparent, resulting in buttons disappearing on hover.

With this PR, styles.css again has the intended colors:

--btn-primary-bg:#063f61;
--btn-primary-bg-hover:#085685;
--btn-primary-bg-active:#0a649b;

Testing done

I tested this change manually against a (proprietary) plugin with a button with these CSS classes: yui-button yui-submit-button submit-button primary. Before the change, the buttons become invisible on hover, and after the change, they show the lighter color on hover as intended.

Proposed changelog entries

• Fix invalid CSS which caused some buttons to become invisible on hover

Proposed upgrade guidelines

N/A

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Desired reviewers

@janfaracik

Before the changes are marked as ready-for-merge:

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[dwnusbaum]

I am far from an SCSS expert, so IDK whether this is the right way to do things, but it does fix the issue.

[janfaracik]

Hey! Thanks so much for raising and implementing the fix. This all looks good to me 👍

I'd recommend adopting the new jenkins-button classes where possible (https://weekly.ci.jenkins.io/design-library/Buttons/) instead of the yui-button classes. It's very much a long term plan but the eventual idea is to reduce/remove usages of YUI from core.

[dwnusbaum]

Ah I suspect that this is also the cause of JENKINS-71479, reported by @NotMyFault. I'll double-check and update the PR title if so.

(EDIT: I confirmed that it is indeed the same issue)

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!