TweetDeck centered

firefox-stylus.user.css

/* ==UserStyle==
@name           tweetdeck-centered
@namespace      tweetdeck.twitter.com
@description    Center TweetDeck's columns on large screens
@version        1.0.3
@author         Corentin Hatte (https://www.github.com/viki53)
@homepageURL    https://gist.github.com/viki53/57f3a84bb629487f0568a741957d475d
@updateURL      https://gist.githubusercontent.com/viki53/57f3a84bb629487f0568a741957d475d/raw/firefox-stylus.user.css
@preprocessor   default
==/UserStyle== */

@-moz-document domain("tweetdeck.twitter.com") {
    .app-columns-container {
        display: flex;
        justify-content: center;
    }

    .app-columns {
        flex: 0 1 auto;
        overflow-x: auto !important;
        height: 100vh !important;
        margin: auto;
    }
}

safari.userscripts.css

/* ==UserStyle==
@name           tweetdeck-centered
@description    Center TweetDeck's columns on large screens
@match          https://tweetdeck.twitter.com/*
@version        1.0.3
@author         Corentin Hatte (https://www.github.com/viki53)
@homepageURL    https://gist.github.com/viki53/57f3a84bb629487f0568a741957d475d
@updateURL      https://gist.githubusercontent.com/viki53/57f3a84bb629487f0568a741957d475d/raw/safari.userscripts.css
==/UserStyle== */

.app-columns-container {
    display: flex;
    justify-content: center;
}

.app-columns {
    flex: 0 1 auto;
    overflow-x: auto !important;
    height: 100vh !important;
    margin: auto;
}