Version 4.8.1, VS2015 using CRISP Theme
Using a dynamic closable TabPanel

The rendering is showing a faint image on top of the close icon...Refer Image attached
The Browser is Chrome (Version 74.0.3729.169 (Official Build) (64-bit))Click image for larger version. 

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It is the same on Microsoft edge

Other Themes do not show the Image

I noted that I can get at the faint Image as ".x-tab-close-btn" and I have set the color to Red to make it clearer.
I have replicated this using the EXT.Net Demo "TabPanel with TabCloseMenu plug in"

It does not appear if the Tabs are not set to closable ...

Sample Page...
[

RM_TestRec1.aspx
<%@ Page Language="VB" AutoEventWireup="false" CodeFile="RM_TestRec1.aspx.vb" Inherits="management_administration_RM_TestRec1" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<!DOCTYPE html>
<html>
<head runat="server">
<title>Scrollable Tabs with TabScrollerMenu Plugin - Ext.NET Examples</title>
<link href="/resources/css/examples.css" rel="stylesheet" />

<%--used to highlight the faint image --%>
<style>
.x-tab-close-btn {
font-size: 14px !important;
color: red;
background-color: red;
}
</style>
</head>
<body>
<form runat="server">
<ext:ResourceManager runat="server" />
<h1>Scrollable Tabs with TabScrollerMenu Plugin</h1>
<ext:TabPanel ID="TabPanel1" runat="server" Height="100">
<Items>
<ext:Panel runat="server" Title="Tab One" Closable="true" />
<ext:Panel runat="server" Title="Tab Two" Closable="true" />
<ext:Panel runat="server" Title="Tab Three" Closable="true" />
<ext:Panel runat="server" Title="Tab Four" Closable="true" />
<ext:Panel runat="server" Title="Tab Five" Closable="true" />
</Items>
</ext:TabPanel>
</form>
</body>
</html>

RM_TestRec1.aspx.vb
Partial Class management_administration_RM_TestRec1
Inherits System.Web.UI.Page

End Class

]