Fix toolbar button contrast by removing opacity

[Saniya-r-Shaikh]

Description

Fixes an accessibility contrast issue in the CMS UI toolbar.

All toolbar buttons were using opacity: 0.8, which reduced contrast and caused them to fail accessibility contrast requirements.
This change removes opacity usage so buttons retain full contrast and readability.

This aligns toolbar buttons with accessibility contrast guidelines by avoiding opacity-based contrast reduction.

Notes

• CSS-only change
• No functional behavior changed
• Tested manually on https://demo.plone.org/edit
• Volto 18.27.3

Closes #7765

[boring-cyborg]

Caution

The Volto Team has suspended its review of new pull requests from first-time contributors until the release of Plone 7, which is preliminarily scheduled for the second quarter of 2026.
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