[AdminUi] RFC: Parametrized `ThemeProvider` to load further `AdminUi` implementation

[maxperei]

hi there,
i came across #296 recently and it made me curious about a new skin for sylius admin and how this could be loaded into the whole thing..

context

as a sylius developer, i wanted to switch between admin themes with a single configuration change so i was wondering about a new parameter in bundle configuration which will be responsible of choosing the resource we want to load custom ui :

# config/packages/sylius_admin_ui.yaml
sylius_admin_ui:
    theme: bootstrap  # or daisyui, shadcn, custom, etc.

as a sylius agency, this also could be useful to deploy multi-tenant projects with different branding requirements without duplicating codebase :

# .env.staging
SYLIUS_ADMIN_THEME=bootstrap

# .env.prod
SYLIUS_ADMIN_THEME=daisyui

# .env.client_acme
SYLIUS_ADMIN_THEME=acme

# config/packages/sylius_admin_ui.yaml
sylius_admin_ui:
    theme: '%env(SYLIUS_ADMIN_THEME)%'

status quo

users can already manage their imports with symfony flex recipes but this does not provide a centralized control for multi-environment setups

proof of concept

i tried to implement a working prototype that demonstrates :

• theme registration via attributes
• dynamic configuration loading via prepend()
• comprehensive unit tests with fixtures
• backward compatibility (existing manual imports still work)

next steps

if it is relevant enough i can:

1. polish the pr with docs and changelog
2. add a migration guide for existing projects
3. create issues for follow-up features (theme:list command, inheritance, theme-specific configuration etc.)

looking forward to your feedback