custom.css

/*

// Override Slack CSS

// Tested with
// - Slack 2.8.2 Direct Download
// - Slack 2.8.2 App Store (Zero)

// Instructions:
// - Append the following javascript code to:
//  - /Applications/Slack.app/Contents/Resources/app.asar.unpacked/src/static/ssb-interop.js (MacOS/X)
//  - /usr/lib/slack/resources/app.asar.unpacked/src/static/ssb-interop.js (Debian/Ubuntu)

//
// The following JavaScript snippet was authored by Jonathan la Cour, https://github.com/lacour
// and released to the public domain under CC0 (https://directory.fsf.org/wiki/License:CC0)
//

document.addEventListener('DOMContentLoaded', function() {
 $.ajax({
   url: 'https://raw.githubusercontent.com/openark/custom-slack-css/master/custom.css',
   success: function(css) {
     $("<style></style>").appendTo('head').html(css);
   }
 });
});

*/

/* Respect PRE: no wrap */
.file_container.snippet_container.snippet_wrap .CodeMirror .CodeMirror-code > div pre,
.file_container.snippet_container.snippet_wrap .sssh-code .sssh-line pre,
.message .file_container.snippet_container .CodeMirror .CodeMirror-code > div pre,
.message .file_container.snippet_container .sssh-code .sssh-line pre {
  white-space: pre;
}

/* Respect PRE: no wrap */
.c-snippet--wrap .CodeMirror-code {
  overflow-x: scroll;
}
.c-snippet--wrap .CodeMirror-code > div > pre {
  white-space: pre;
}
pre.special_formatting {
  white-space: pre;
  overflow-x: scroll;
}

/* Bot messages: fixed font */
/* .c-message is 3.0 notation */
span.c-message_attachment__text,
.c-snippet__code,
ts-message[data-bot-id] .message_body .msg_inline_attachment_column {
  font-family: Consolas,monaco,"Ubuntu Mono",courier,monospace;
  font-size: 13px;
  line-height: 20px;
}

/* Bot messages: ovrride 600px width limitation ; highlight */
/* .c-message is 3.0 notation */
.c-message .c-message__attachments,
.c-message .c-message_attachment {
  max-width: inherit;
}
.c-message .c-message_attachment__body {
  color: #333333;
  background-color: #f9f9f9;
}
/* 2.X */
ts-message[data-bot-id] .message_body .attachment_group {
  max-width: inherit;
  color: #333333;
  background-color: #f9f9f9;
}

/* Bot messages: ovrride image size limitation */
ts-message[data-bot-id] .message_body .attachment_group .file_preview_preserve_aspect_ratio {
    width: 100% !important;
    height: 100% !important;
}