How to change background color prefix description?

Hi,

is a prefix for thread (phrase: thread_prefix_desc.1)

In the Browser Inspector:
CSS:

.blockMessage.blockMessage--alt {
    color: #141414;
    background: #f5f5f5;
}

I saw the thread about style (extra.less) with:
CSS:

.blockMessage--close.prefixDesc--id2 {
    border-left: 3px solid #d31117;
}

I tried but I couldn't

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