fix(web-app-password-protected-folders): [OCISDEV-917] fix theme switching and cancel button visibility

[mzner]

Description

Fix theme switching issues where colors could get stuck or become unreadable until a page refresh. Additionally fix the cancel button in the password protected folder modal being invisible against the dark action bar background.

Changes:

• Clear previous theme's custom properties before applying the new theme so stale values don't persist
• Treat empty token strings as unset by removing the CSS property instead of setting it to blank
• Style the cancel button in FolderViewModal to use brand contrast colors for visibility

Related Issue

• Fixes OCISDEV-917
• Fixes OCISDEV-910
• Fixes [OCISDEV-941] Password Protected Folder Close folder button is hard to see #13817

Motivation and Context

When switching between themes, tokens from the previous theme were not cleared before applying the new theme, causing stale values to persist. Empty string values in theme tokens were overriding stylesheet defaults with nothing, making elements invisible. The cancel button in the password protected folder modal was invisible because its color matched the dark action bar background.

How Has This Been Tested?

• test environment: local dev server with oCIS backend
• test case 1: Switch between light and dark themes — verify no stale color values persist
• test case 2: Open password protected folder modal — verify cancel button is visible and has proper hover/focus states

[kw-security]

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[sonarqubecloud]

Quality Gate failed

Failed conditions
41.0% Condition Coverage on New Code (required ≥ 50%)

See analysis details on SonarQube Cloud