Cannot custom CSS style on post grid

Hi. I cannot change the color and style for the filter navigation of your post grid block. That should be something like:

.ultp-block-5288b4 .ultp-filter-navigation .ultp-filter-wrap ul li a {
    color: #ffffff;
}

However the .ultp-block selector changes every time. I try to add custom CSS, but it never stays there. the block setting tab says:

Add {{ULTP}} before the selector to wrap
element.

But I don’t understand what should I type then.

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