[ENH] - Override theme when using jhub-apps

[aktech]

Feature description

The way styling works at the moment is via:

nebari/src/_nebari/stages/kubernetes_services/template/modules/kubernetes/services/jupyterhub/files/jupyterhub/01-theme.py

Lines 14 to 17 in f04c70d

	if z2jh.get_config("custom.jhub-apps-enabled"):
	from jhub_apps import themes
	c.JupyterHub.template_vars = {**jupyterhub_theme, **themes.DEFAULT_THEME}

if z2jh.get_config("custom.jhub-apps-enabled"):
    from jhub_apps import themes

    c.JupyterHub.template_vars = {**jupyterhub_theme, **themes.DEFAULT_THEME}

The current flow is:
- Use default theme param or user supplied (from nebari config) params when not using jhub apps
- Use jhub apps theme when using jhub apps

We need to provide a priority/mechanism to override jhub-apps theme params when using jhub-apps.

Value and/or benefit

To be able to customise colors based on brand colors of the organization.

Anything else?

No response

[aktech]

Fixed in #2215