Midnight Twitter - A Stylish style that adds some light to the default "lights out" theme of Twitter.

midnight-twitter.css

/* Background, tweet background & panels on the right. */
body, .r-kemksi, .r-1igl3o0, .r-9qu9m4 {
    background-color: rgb(33, 36, 38) !important;
}

/* Tweet background (hovered). */
.r-1ysxnx4 {
    background-color: rgb(48, 53, 58) !important;
}

/* Divider and search bar. */
.r-z32n2g {
    background-color: rgb(55, 60, 65) !important;
}

/* Blue. */

/* Button. */
.r-urgr8i {
    background-color: rgb(53, 148, 207) !important;
}

/* Bordered button. */
.r-p1n3y5 {
    border-bottom-color: rgb(53, 148, 207) !important;
    border-left-color: rgb(53, 148, 207) !important;
    border-right-color: rgb(53, 148, 207) !important;
    border-top-color: rgb(53, 148, 207) !important;
}

/* Link. */
.r-13gxpu9 {
    color: rgb(53, 148, 207) !important;
}

/* Yellow. */

/* Button. */
.r-1vkxrha {
    background-color: rgb(211, 157, 60) !important;
}

/* Bordered button. */
.r-v6khid {
    border-bottom-color: rgb(211, 157, 60) !important;
    border-left-color: rgb(211, 157, 60) !important;
    border-right-color: rgb(211, 157, 60) !important;
    border-top-color: rgb(211, 157, 60) !important;
}

/* Link. */
.r-61mi1v {
    color: rgb(211, 157, 60) !important;
}

/* Pink. */
/* Button. */
.r-1dgebii {
    background-color: rgb(189, 61, 104) !important;
}

/* Bordered button. */
.r-1iofnty {
    border-bottom-color: rgb(189, 61, 104) !important;
    border-left-color: rgb(189, 61, 104) !important;
    border-right-color: rgb(189, 61, 104) !important;
    border-top-color: rgb(189, 61, 104) !important;
}

/* Link. */
.r-daml9f {
    color: rgb(189, 61, 104) !important;
}

/* Purple. */

/* Button. */
.r-1qqlz1x {
    background-color: rgb(117, 88, 175) !important;
}

/* Bordered button. */
.r-11mmphe {
    border-bottom-color: rgb(117, 88, 175) !important;
    border-left-color: rgb(117, 88, 175) !important;
    border-right-color: rgb(117, 88, 175) !important;
    border-top-color: rgb(117, 88, 175) !important;
}

/* Link. */
.r-xfsgu1 {
    color: rgb(117, 88, 175) !important;
}

/* Orange. */

/* Button. */
.r-18z3xeu {
    background-color: rgb(203, 101, 62) !important;
}

/* Bordered button. */
.r-1xl5njo {
    border-bottom-color: rgb(203, 101, 62) !important;
    border-left-color: rgb(203, 101, 62) !important;
    border-right-color: rgb(203, 101, 62) !important;
    border-top-color: rgb(203, 101, 62) !important;
}

/* Link. */
.r-1qkqhnw {
    color: rgb(203, 101, 62) !important;
}

/* Green. */

/* Button. */
.r-b5skir {
    background-color: rgb(49, 169, 107) !important;
}

/* Bordered button. */
.r-5ctkeg {
    border-bottom-color: rgb(49, 169, 107) !important;
    border-left-color: rgb(49, 169, 107) !important;
    border-right-color: rgb(49, 169, 107) !important;
    border-top-color: rgb(49, 169, 107) !important;
}

/* Link. */
.r-nw8l94 {
    color: rgb(49, 169, 107) !important;
}

Before applying this style, open Twitter, on the left side click "More" - "Display" and select the "Lights out" theme. All of the 6 colors are supported.

Screenshot of the purple color:

... and the original style: