How to change the message background color?

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Changing the message background color include, changing the background, changing the text color and changing the tip at the message (the small triangle pointing to the user image).

.message-bubble { background-color: #333333 !important; }
.message-text { color: #ffffff; }
.main-message-container::after {
border-left-color: #333333;
border-right-color: #333333;

To know how to edit the mobile CSS, refer to this link.