GH-10: Document the usage of theme variables

Author: ArtyomVancyan

README.md

@@ -15,7 +15,7 @@ python -m pip install django-mermaid
 
 ## Configuration
 
-Add the `django_mermaid.apps.MermaidConfig` to your `INSTALLED_APPS` setting:
+Add the `django_mermaid.apps.MermaidConfig` to your `INSTALLED_APPS` in your Django project's **settings.py** file.
 
 ```python
 INSTALLED_APPS = [
@@ -26,7 +26,8 @@ INSTALLED_APPS = [
 
 ## Usage
 
-Once you have installed the app, you can use the `mermaid` template tag in your templates.
+After you configure the `INSTALLED_APPS`, you will be able to load the `mermaid` in your template and use the template
+tag for rendering mermaid diagrams.
 
 ```jinja2
 {% load mermaid %}
@@ -47,23 +48,37 @@ the **mermaid.min.js** file.
 
 ### Mermaid theme
 
-You can specify the theme right in the template tag dynamically overriding the value of the `MERMAID_THEME` variable.
+To set a default theme for the whole project, set the `MERMAID_THEME` variable in your Django project's **settings.py**
+file. However, Django Mermaid uses
+the [default](https://github.com/mermaid-js/mermaid/blob/develop/packages/mermaid/src/themes/theme-default.js) theme of
+mermaid by default. Also, check out the mermaid [docs](https://mermaid.js.org/config/theming.html?#theme-variables) for
+the available themes.
+
+```python
+MERMAID_THEME = 'neutral'
+```
+
+Also, you can provide the theme right in the template tag which will dynamically override the value of
+the `MERMAID_THEME` variable.
 
 ```jinja2
 {% mermaid "graph LR; A-->B;" "dark" %}
 ```
 
-Also, you can set the `MERMAID_THEME` variable in your Django project's **settings.py** file as a default theme.
-However, Django Mermaid uses the
-[default](https://github.com/mermaid-js/mermaid/blob/develop/packages/mermaid/src/themes/theme-default.js) theme of
-mermaid by default.
+### Mermaid theme variables
+
+You can define your custom theme by overriding the `MERMAID_THEME_VARIABLES` variable. You will need to use
+the [base](https://github.com/mermaid-js/mermaid/blob/develop/packages/mermaid/src/themes/theme-base.js) theme as it is
+the only modifiable theme. Check out the mermaid [docs](https://mermaid.js.org/config/theming.html?#theme-variables) for
+the complete and up-to-date list of available theme variables.
 
 ```python
-MERMAID_THEME = 'neutral'
+MERMAID_THEME_VARIABLES = {
+    'primaryColor': '#BB2528',
+    'primaryTextColor': '#FFF',
+}
 ```
 
-The available themes supported by mermaid are listed at mermaid [docs](https://mermaid.js.org/config/theming).
-
 ## Contribute
 
 Any contribution is welcome. If you have any ideas or suggestions, feel free to open an issue or a pull request. And