Fix some translations in Preferences

Hello team,

Two translations in 

src\app\pages\preferences\page\forms\general-preferences-form.component.ts

src\app\pages\preferences\page\forms\custom-theme-manager-form.component.ts

not work. T marker & PO file are OK.

1. General Preferences

2. Manage Custom Themes

public fieldSets: FieldSet[] = [

    {

      name:T('General Preferences'),

      class:'preferences',

      label:true,

      config: []

    }

  ]

public fieldSets: FieldSet[] = [

      {

        name:T('Manage Custom Themes'),

        class:'theme-manager',

        width:'100%',

        label:true,

        config:this.customThemeFields

      }

    ]