docs: add primary_color config option to web.md

docs/web.md

@@ -12,6 +12,7 @@ web:
   enable: true
   title: Verdaccio
   logo: logo.png
+  primary_color: "#4b5e40"
   gravatar: true | false
   scope: @scope
   sort_packages: asc | desc
@@ -28,7 +29,8 @@ title | string | No | Verdaccio | all | HTML head title description
 gravatar | boolean | No | true | `>v4` | Gravatars will be generated under the hood if this property is enabled
 sort_packages | [asc,desc] | No | asc | `>v4` | By default private packages are sorted by ascending
 logo | string | No | http://my.logo.domain/logo.png | all | a URI where logo is located (header logo)
+primary_color | string | No | "#4b5e40" | `>4` | The primary color to use throughout the UI (header, etc)
 scope | string | No | \\@myscope | `>v3.x` | If you're using this registry for a specific module scope, specify that scope to set it in the webui instructions header (note: escape @ with \\@)
 
 
-> It is recommended the logo size has the following size `40x40` pixels.
+> It is recommended the logo size has the following size `40x40` pixels.