Put this on obs Custom CSS field on the browser source of the chat. URL used, replace CHANNEL: https://nightdev.com/hosted/obschat?theme=undefined&channel=CHANNEL&fade=false&bot_activity=false&prevent_clipping=false

kap.css

/* Just transparent background and text-shadow */
body {
  background-color: rgba(0, 0, 0, 0);
  margin: 0px auto;
  overflow: hidden;
  text-shadow: 1px 1px 0px #000;
}

streamLabsClean.css

/* These rules try to mimic the Stream Labs default Clean theme on KapChat*/

html {
  font-size: 20px;
  width: 100%;
  height: 100%;
}
body {
  overflow: hidden;
  margin: 0.313rem;
}
#chat_box {
  background-color: transparent;
  font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
  font-size: 0.813rem;
  font-style: normal;
  font-variant: normal;
  font-weight: normal;
  position: absolute;
  overflow: hidden;
  color: #ffffff;
  border-radius: 0.25rem;
  width: calc(100% - 0.625rem);
}
#chat_box.dark {
  background: rgba(0, 0, 0, 0.3);
  color: #fff;
}
#chat_box.light {
  background: rgba(255, 255, 255, 0.3);
  color: #000;
}
.nick {
  font-weight: bold;
}

.tag {
  display: inline-block;
  text-indent: 1.313rem;
  background-position: 0 center;
  background-repeat: no-repeat;
  background-size: contain;
  display: inline-block;
  vertical-align: bottom;
  height: 1.125rem;
  min-width: 1.125rem;
  padding: 0;
  margin-right: 0.188rem;
  margin-bottom: -0.063rem;
  text-indent: -624.938rem;
  border-radius: 0.125rem;
  -moz-border-radius: 0.125rem;
  -webkit-border-radius: 0.125rem;
  overflow: hidden;
  vertical-align: sub; /* aligns with the middle of text */
}
.chat_line {
  margin-left: 0.188rem;
  margin-right: 0.188rem;
  padding-top: 0.125rem;
  padding-bottom: 0.188rem;
  line-height: 1.25rem; /* avoid clipping */
}
.chat_line .message {
  word-wrap: break-word;
}
.chat_line .time_stamp {
  display: none;
  padding-right: 0.188rem;
}
.emoticon {
  margin-bottom: -0.438rem;
  min-width: 1.75rem; /* min dimensions enforced to some scale of 18x18 */
  min-height: 1.75rem;
  vertical-align: sub; /* aligns with the middle of text */
}
.cheermote {
  width: auto;
  max-height: 1.75rem;
}
.chat_line {
  text-shadow: -1px -1px 0px #000, 1px -1px 0px #000, -1px 1px 0 #000,
    1px 1px 0 #000;
  letter-spacing: 0.06rem; /* letter spacing in an attempt to combat crowding created by text-shadow */
}
.chat_line[data-nick="streamelements"] {
  display: none; /* hide streamelements messages */
}
.chat_line {
  animation: fadeInRight 0.3s ease forwards;
}

@keyframes fadeInRight {
  0% {
    opacity: 0;
    -webkit-transform: translate3d(100%, 0, 0);
    transform: translate3d(100%, 0, 0);
  }

  to {
    opacity: 1;
    -webkit-transform: none;
    transform: none;
  }
}

See also https://gist.github.com/Fire-/9f862fd55f624481bfebc3dfde638971 and https://gist.github.com/Fire-/984ec956ced8ac875ba7600da689127d for other styles