How can I use Preprocessor Directives to Conditionally Include Specific CSS Snippets?

Problem:

• I have two build configurations. Each has its own preprocessor symbol: DEV and PROD.
• I would like each build to have its own background-image color.
• For C# files, I know that I can use preprocessor directives with the build symbols to conditionally generate code.

Question: How can I do this with a CSS file (like below)?

.page {
    position: relative;
    display: flex;
    flex-direction: column;
}

main {
    flex: 1;
}

.sidebar {
#if PROD
    background-image: linear-gradient(180deg, #ff80ed 0%, gainsboro 90%);
#else
    background-image: linear-gradient(180deg, rgb(5, 39, 103) 0%, #3a0647 70%);
#endif
}

UPDATE:

• Alternatively, I'd be just as happy creating a site.css.dev and site.css.prod if that would solve the problem.

Inspired by this answer:

1. Remove the .sidebar code from MainLayout.razor.css.
2. Add two new files in the wwwroot\css directory:
   1. sidebar.Default.css

.sidebar {
    background-image: linear-gradient(180deg, rgb(5, 39, 103) 0%, #3a0647 70%);
}

2. sidebar.Prod.css

.sidebar {
    background-image: linear-gradient(180deg, #ff80ed 0%, gainsboro 90%);
}

3. Modify _Layout.cshtml as follows:

    <link href="css/site.css" rel="stylesheet" />
        @{
            var isProd = false;
            #if PROD
            isProd = true;
            #endif
        }
        @if (isProd)
        {
            <link href="css/sidebar.Prod.css" rel="stylesheet" />
        }
        else
        {
            <link href="css/sidebar.Default.css" rel="stylesheet" />
        }
        <link href="FooBar.styles.css" rel="stylesheet" />