The background colour of the text area, date and vertical line can be edited in this section of CSS:

/*Add the background to the post content*/

@media all and (min-width: 601px) {
    .ds-advent-9 .et_pb_post:before {
        content: '';
        height: 100%;
        background: #dedede;
        position: absolute;
        left: 45%;
        right: 0;
        top: 0;
        bottom: 0;
        z-index: -1;
        -webkit-transition: all .3s ease;
        transition: all .3s ease;
    }
    /*Add the vertical line below the date*/
    .ds-advent-9 .et_pb_post .published:before {
        left: 50%;
        top: 100%;
        content: " ";
        height: 200%;
        width: 5px;
        position: absolute;
        background: #dedede;
        -webkit-transition: all .3s ease;
        transition: all .3s ease;
    }
}

The colour of the arrow next to the date can be edited in this section of CSS:

/*Add the side arrow to the date*/

.ds-advent-9 .et_pb_post .published:after {
    left: 100%;
    top: 50%;
    border: solid transparent;
    content: " ";
    height: 0;
    width: 0;
    position: absolute;
    z-index: 1;
    pointer-events: none;
    border-color: transparent;
    border-left-color: #dedede;
    border-width: 15px;
    margin-top: -15px;
    -webkit-transition: all .3s ease;
    transition: all .3s ease;
}

The colour on hover can be edited in this section of CSS:

/*Change the arrow colour on hover*/

.ds-advent-9 .et_pb_post:hover .published:after {
    border-left-color: #8ACAF8;
}


/*Change the date, vertical line and post content background colour on hover*/

.ds-advent-9 .et_pb_post:hover .published,
.ds-advent-9 .et_pb_post:hover .published:before,
.ds-advent-9 .et_pb_post:hover:before {
    background: #8ACAF8;
}

Also check the CSS under /*Rearrange content for mobiles*/ for the colours on smaller screens.