Use project default theme for dialogic defaults

[ryan-linehan]

Is your feature request related to a problem? Please describe.
When setting up theming in dialogic it can be really confusing to use the custom scenes for a theme. In order for me to utilize my godot project's theme that the rest of my game's UI uses the following steps need to be taken:

1. Add a premade style > Visual Novel Style
2. Click style > click Custom Visual Novel Textbox > Click make custom button > current layer
3. Click to open the scene that was created (and a lot of other resources!)
4. Click the root node > Inspector panel > Box > Panel > uncheck box use global color
5. Inspector panel > Box > Panel > box color custom > set to #fff (godot's default when using modulate which this property seemingly sets)
6. Inspector panel > Box > Panel > Box Panel and remove this value all together (this will have the project throw errors since it expects it but since it doesn't have it the controls just use the project's default)
7. Click DialogTextPanel in scene tree remove the theme override that was set (so the control node uses your default project theme)8. Lastly , still on DialogTextPanel reset self modulate and now your godot project theme utilized
   In general I think a user utilizing dialogic would want the default theming to match what they have set their UI up as for the rest of their game. If they don't want to go that way with it then they could just add a another godot theme and utilize each control node's theme override properties to let the custom scene drive what it looks like.

It is also unclear if the layers tab actually affects anything when you've create a custom scene theme. Does it? It was so difficult to tell because there are 3 or 4 layers of these things being overwritten by different settings. Global settings, the dialogic panel settings, the root node of a custom layer, the children nodes of a custom layer all seemingly could control the same or similar things. Since we're just in the godot's scene editor anyways why not have the nodes behave more naturally in here instead of all these overrides?

Describe the solution you'd like
Dialogic theming defaults to how your godot project's theme or an option when creating a custom layer scene for it to default to what your project's theme is (or even just specify the file to use). There also wouldn't be a need to create a the resources that are created when you make a custom layer because the godot user would be making these anyway for their custom theme (or will have already made them).

Describe alternatives you've considered
I'm working around it by figuring out how to untangle the defaults setup by dialogic even when setting up a custom layer scene

Additional context
my default theme setup in the godot project settings + it working in dialogic + the theme editor at the bottom of the screen

screenshot of scene tree with root node highlighted for inspector setup

[Nao-fox]

I appreciate that you provided a clear explanation about how to set up a custom style.
The default theme configuration of this plugin is quite complicated and unclear... I struggled with it for hours. The way it applies both overriding theme and using global color by default doesn't really make sense, since most users typically prefer custom-made themes. In most cases, these global color settings end up being unnecessary and confusing.

Your explanation should definitely be included in the official tutorial, or the theme configuration itself should be simplified.