dashboard widgets are configurable via config (#269)

var/docs/theme.md

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+# Theme
+
+Kimai uses the [AdminLTE theme](https://github.com/kevinpapst/AdminLTEBundle/) which can be configured in the file `config/packages/admin_lte.yaml`. 
+You find the theme specific documentation [here](https://github.com/kevinpapst/AdminLTEBundle/blob/master/Resources/docs/configurations.md).
+
+All Kimai specific theme settings will be available in the twig templates with the global `kimai_context` key, e.g.
+
+```twig
+{{ kimai_context.box_color }}
+``` 
+
+## Active entries warning
+
+A small colored warning sign will be shown, if a user has more than 3 active timesheet entries.
+
+You can change this soft limit by setting the config key `kimai.theme.active_warning` in your `local.yaml`:
+
+```yaml
+kimai:
+    theme:
+        active_warning: 2
+```
+
+## Colors
+
+Kimai allows you to configure colors in several places throughout the theme. 
+
+Possible values are:
+
+- `aqua`
+- `black`
+- `blue`
+- `gray`
+- `green`
+- `purple`
+- `red`
+- `yellow`
+
+### Fallback color
+
+Whenever a color is required but none is configured, Kimai uses a fallback language from the config key `kimai.theme.box_color`.
+
+You can change the default color `green` to any one from the above in your `local.yaml`:
+
+```yaml
+kimai:
+    theme:
+        box_color: 'blue'
+```
+
+The fallback color should be applied whenever an optional color is configurable by the user:
+
+```twig
+<div class="info-box bg-{{ color|default(kimai_context.box_color) }}"></div>
+```
+
+## Icons
+
+Kimai allows you to configure icons in several places (provided by [Font Awesome 5](https://fontawesome.com/icons)) and ships 
+with a pre-defined list of icon aliases to guarantee a consistent look.  
+
+The pre-defined icons aliases are:
+
+- `user`
+- `customer`
+- `project`
+- `activity`
+- `admin`
+- `invoice`
+- `timesheet`
+- `dashboard`
+- `logout`
+- `trash`
+- `delete`
+- `repeat`
+- `edit`
+- `manual`
+- `help`
+- `start`
+- `start-small`
+- `stop`
+- `stop-small`
+- `filter`
+- `create`
+- `list`
+- `print`
+- `visibility`
+- `calendar`
+- `money`
+- `duration`
+
+Icon aliases can be used by applying the `icon` filter, e.g.
+
+```
+<i class="{{ 'money'|icon }}"></i>
+```
+
+