CSS a11y button fixes breaking theming flexibility

[LotteHofstede]

Bootstrap style overrides break theming

In _global-styles.scss, success and warning button colors are manually darkened to meet a11y requirements:

• btn-success / btn-outline-success: darkened via darken($success, …)
• btn-warning / btn-outline-warning: darkened via darken($warning, …)
• Success button text color is hardcoded to #fff

File reference:
https://github.com/DSpace/dspace-angular/blob/main/src/styles/_global-styles.scss#L363

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Why this is a problem

• Breaks theming: custom $success / $warning colors are always overridden, even if already a11y compliant
• Inconsistent across components: buttons use altered colors, while other components (badges, alerts, etc.) use base values
• Bypasses Bootstrap color system: semantic colors should be defined once and reused, not modified per variant
• Contrast handling becomes unreliable: hardcoded color: #fff bypasses Bootstrap’s contrast logic (color-contrast() in BS5)

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Root cause

Introduced in:
#2587

Accessibility fixes were applied to specific Bootstrap button variants instead of adjusting semantic theme colors.

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Proposed solution

• Define accessible $success and $warning colors at theme level
• Remove darken(...) overrides from button styles
• Remove hardcoded color: #fff
• Let Bootstrap handle contrast (color-contrast() in BS5, or $yiq-contrast in BS4 for backports)

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Expected outcome

• Consistent behavior across components
• Predictable theming
• Alignment with Bootstrap’s color system