to-do list block : custom text properties for checked item

[LaurentAtJoes]

Hi all,
I'm trying to add a bit of custom css to the default theme. Disclaimer: I'm no CSS expert.

I can't figure out how to set custom color and text-decoration only for CHECKED ITEMS .

If I try out this css code, it obviously sets those text properties for all items. (the part for the checked box background works well)

.ck-editor__editable.ck-content .todo-list .todo-list__label>span[contenteditable=false]>input[checked]:before {
    background: #666666 !important;
    border-color: #222222 !important;
}
.ck-content .todo-list .todo-list__label .todo-list__label__description {
    color: #c9c9c9 !important;
    text-decoration: line-through !important;
}

I assume there maybe a way similar to what is used for the checkbox itself (when checked), with the >span[contenteditable=false]>input[checked]:before part, but I have no idea how to write that.

This is the desired result : (here with manual text formatting, for the example)

Any help would be appreciated