[BUG] theme:darkMode cause security plugin login page blank

[Flyingliuhub]

Describe the bug
theme:darkMode cause security plugin login page blank, when added "theme:darkMode": true or false in the OpenSearch_dashboards yaml file, and installed security plugin, the security plugin's login page show blank.

uiSettings:
  overrides:
    "theme:next": true
    "theme:darkMode": true // or false

To Reproduce
Steps to reproduce the behavior:

1. Use main branch for OSD
2. Use 3..0.0 OS snapshot
3. Install Security dashboard plugin
4. nav to localhost:5601
5. login page show blank

Expected behavior
A clear and concise description of what you expected to happen.

OpenSearch Version
3.0.0

Dashboards Version
main branch - 3.0.0

Plugins

Security dashboards plugin

Screenshots

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Host/Environment (please complete the following information):

• OS: [e.g. iOS]
• Browser and version [e.g. 22]

Additional context

Add any other context about the problem here.

[derek-ho]

This seems unrelated to security plugin, as I am on an older version of OSD and there is no issue. There seems to be similar issues recently with CSS/themes being undefined, for example: #6703

[manasvinibs]

We have a fix for this issue in this PR #6730