Write JavaScript statements in ASP.net's back code to prevent hint box background from becoming white

In general, when we pop a message like MsgBox in a Web page, the background page is white, which makes it look bad.
For example:

How to solve this problem?
You can write JavaScript statements in ASP.net's back code to prevent the background from becoming white
if (Code!=rightcode)
{
Page.ClientScript.RegisterStartupScript (Page.gettype (), "message", "");
After this, the effect is:

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