Simple style for a clean Trello interface. https://userstyles.org/styles/198159/trello-declutterd

clean-trello.css

._1CyMivLdH2a8dA, ._32kNZW8b-DAcUd, ._3Z6i0FBUukKNYK, ._27QKbGhxDkdNHq, ._2x-vl1QLZ6Q3-O {
    display: none;
}

.home-left-sidebar-container {
    display: none;
}
.boards-page-board-section-header-options {
    display: none;
}
.js-power-up-slots-container {
    display: none;
}
/*.board-header {
    display: none;
}*/

.board-header-btn:not(.no-edit):not(.board-header-filter-btn-active) {
    background-color: transparent;
}

.js-plugin-header-btns {
    display: none;
}

.org-label {
    display:none;
}

.board-header-btns .member {
    display: none;
}
.board-header-btn-invite {
    margin-left: 0;
}

.board-menu-navigation-item.disabled {
    display: none;
}
.quiet {
    display: none;
}
.boards-page-ad-section, .ads-layout-experiment {
    display: none;
}

._1SbjnCnwan4fsg * {
    text-decoration: none;
}

._2nAwZM22FU-LKV {
    display: none;
}
button {
    text-decoration: none !important;
}

a {
    text-decoration: none;
}
.js-billing {
    display: none;
}

This is a style you can install using Stylish.
Download link here: https://userstyles.org/styles/198159/trello-declutterd