.fromjs-value span:hover {
    color: lime
}
.fromjs-value {
    padding: 10px;
}

#fromjs * {
    box-sizing: border-box;
}

.fromjs-highlighted-character {
    color: #0088ff;
    font-weight: bold;
    border-bottom: 2px solid #0088ff;
}

#fromjs {
    position: fixed;
    font-family: "Arial";
    font-size: 16;
    right: 0;
    top: 0;
    bottom: 0;
    height: 100vh;
    width: 40vw;
    background: white;
    overflow: auto;
    border-top: 1px solid black;
}

body {
    padding-right: 40vw;
}

.fromjs-stack__line-number {
    border-right: 1px solid #ddd;
    padding-right: 5px;
    margin-right: 5px;
    text-align: right;
    min-width: 30px;
    display: inline-block;
}

.fromjs-horizontal-scroll-container {
    overflow-x: auto;
    white-space: nowrap;
}
.fromjs-horizontal-scroll-container > div {
    display: inline-block;
    min-width: 100%;
}

.fromjs-btn-link{
    color: green;
    cursor: pointer;
}
.fromjs-btn-link:hover {
    text-decoration: underline;
}

.fromjs-value span:hover {
    color: red;
    border-bottom: 2px solid red;
}
.fromjs-value {
    padding: 10px;
}
.fromjs-value span {
    cursor: pointer;
}

#fromjs * {
    box-sizing: border-box;
}

.fromjs-highlighted-character {
    color: #0088ff;
    font-weight: bold;
    border-bottom: 2px solid #0088ff;
}

#fromjs {
    position: fixed;
    font-family: "Arial";
    font-size: 16;
    right: 0;
    top: 0;
    bottom: 0;
    height: 100vh;
    width: 40vw;
    background: white;
    overflow: auto;
    border-top: 1px solid black;
}

body {
    padding-right: 40vw;
}

.fromjs-stack__line-number {
    border-right: 1px solid #ddd;
    padding-right: 5px;
    margin-right: 5px;
    text-align: right;
    min-width: 30px;
    display: inline-block;
}

.fromjs-horizontal-scroll-container {
    overflow-x: auto;
    white-space: nowrap;
}
.fromjs-horizontal-scroll-container > div {
    display: inline-block;
    min-width: 100%;
}

.fromjs-btn-link{
    color: green;
    cursor: pointer;
}
.fromjs-btn-link:hover {
    text-decoration: underline;
}

Tweak char hover highlight styles.
Tweak char hover highlight styles.
