fix(theming): add custom theme support for external CSS

[nnaydenow]

Now, we support two scenarios where OpenUI5/ SAPUI5 is detected and stand-alone web components project.

If OpenUI5Support is feature is registered, we leave previous implementation where sap.ui.core CSS variables should be imported (data-sap-ui-xx-cssVariables should be used in that scenario). CSS variables will be loaded from url + UI5/sap/ui/core/themes/custom_theme/css_variables.css

If OpenUI5Support feature is not registered we load custom CSS variables and rerender all components. CSS variables will be loaded from url + Base/baseLib/custom_theme/css_variables.css

Like in OpenUI5 when CSS variables are loaded from different origin than current, meta tag with allowed theme roots should be used (sap-allowedThemeOrigins).

• Checks for theme origin
• Should we extend the theme origin with UI5?

[vladitasev]

The proposed solution only solves the "theming" package CSS Variables ( the official ones @sapSomething), however we must also load the correct geometric variables for the packages such as @ui5/webcomponents and @ui5/webcomponents-fiori, depending on the base theme, upon which the custom theme passed with the @ syntax was crated in Theme designer.

So the full solution would be something like this: once the custom theme has loaded, use the functions in packages/base/src/theming/getThemeDesignerTheme.js to detect the base theme upon which the custom theme was built, and then trigger loading the CSS Vars for all registered packages.

Example: my custom @-syntax theme loads, then the framework determines it was based on sap_horizon, and for all currently loaded packages, also load the CSS vars for this base theme.

[vladitasev]

IMO boot.js should not have knowledge of this functionality, if possible.

Instead, I would suggest to hook this attachStylesToHead code to the applyTheme function, which is called a few lines below. If you do this, this will also automatically trigger when calling setTheme, as it calls applyTheme internally, so you won't need to change that function either.