Fix Blocks Filter Not Showing Placeholder in Off-Canvas or Layout Builder Modal

Problem/Motivation

Blocks filter in layout builder is shown fully white so the placeholder doesn't show.

Steps to reproduce

- Open any page with layout builder with dark theme.
- Choose add block in the layout builder.
- Notice that the filter bellow add a custom block doesn't show a placeholder

Before the fix:

Proposed resolution

• Add &::placeholder with style to all input form elements in #drupal-off-canvas or #layout-builder-modal
• Switch to use var(--input-fg-color--placeholder) Claro's CSS3 variable.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ✅ Accessibility and Readability
• ✅ Code review from 1 Varbase core team member
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes and Update Helper on new feature change/addition
• ✅ Release varbase-10.0.3, varbase_layout_builder-10.1.4, varbase-9.1.7, varbase_layout_builder-10.0.57

Varbase update type

• ✅ No Update
• ➖ Optional Update
• ➖ Forced Update
• ➖ Forced Update if Unchanged

User interface changes

After the fix:

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Issue #3480375 by oways23, alshami: Fixed Blocks Filter Not Showing Placeholder in Off-Canvas or Layout Builder Modal