Add a Style Configurator's forgotten feature

Add the description of ignoring bg/fg coulor feature which is not well known by the most of users. 
I've forgotten this feature but thanks the PR which makes markdown UDL work in every dark themes (notepad-plus-plus/notepad-plus-plus#10619), we found this feature back.

Author: donho

content/docs/preferences.md

@@ -441,7 +441,7 @@ The "Language:" selection list lets you select whether you want to set the forma
 
 On all but "Language: Global Styles", there will also be a "Default ext." box, which is an un-editable list of the default file extensions associated with that Language; and the "User ext." box, where you can add a user-defined list of additional extensions (space separated, don't use the . in the extension), which says which other extensions you want to apply this language's formatting to.  There is no specific entry called "Normal text" or "Plain text": to edit the colors for a plain text file (like `.txt`), use the "Global Styles" language.  (Please note that any changes you make in the Style Configurator dialog box for a specific language only applies to that language and only applies to the selected [theme](../themes/): if you add a user-defined extension to a language in the `Default (stylers.xml)` theme, it will not affect the list of user-defined extensions for that language in any of the other themes.)
 
-The final section will reiterate which language and style are selected, and allow you to set colors and fonts.  The Colour Style allows you to choose the Foreground or Background colour by clicking on the colored box.  The Font Style allows you to pick the font, size, and bold/italic/underline settings.  If Font name or Font size are left blank, they will inherit from the Global Styles: Default Style.  If you right-click a colour, you will see diagonal stripes across the colour, indicating it is set to "inherit", meaning that it will take that colour from the Default Style.  Under the "Language: Global Styles" with "Style: Global override", there are also a series of checkboxes for "Enable global xxx", which will mean that Notepad++ will use the Global override setting for that attribute, rather than using the per-language styling settings for that attribute.
+The final section will reiterate which language and style are selected, and allow you to set colors and fonts.  The Colour Style allows you to choose the Foreground or Background colour by clicking on the colored box. You can also make the style's Foreground or Background colour ignored, by right-clicking on the colored box (you'll see the additional diagonal stripes on the colored box) - in this case the default background/foreground colour will be used.  The Font Style allows you to pick the font, size, and bold/italic/underline settings.  If Font name or Font size are left blank, they will inherit from the Global Styles: Default Style.  If you right-click a colour, you will see diagonal stripes across the colour, indicating it is set to "inherit", meaning that it will take that colour from the Default Style.  Under the "Language: Global Styles" with "Style: Global override", there are also a series of checkboxes for "Enable global xxx", which will mean that Notepad++ will use the Global override setting for that attribute, rather than using the per-language styling settings for that attribute.
 
 Some language/style combinations (like Perl > INSTRUCTION WORD) will additionally have a list of default keywords (not editable) and user-defined keywords (which allow you to add new keywords to apply this style to).