https://web.telegram.org - via Magic CSS extension for Chrome - https://chrome.google.com/webstore/detail/ifhikkcafabcgolfjegfcgloomalapol

styles.css

.navbar-header,.im_send_panel_wrap,.tg_head_peer_menu_wrap, .im_message_unread_split {
    background-color: rgb(50,40,60);    
}

html, body, .im_history_message_wrap,
.im_dialog {
    background-color: rgb(20,10,30);
    color: #e0dee2 !important;
    
}

.im_dialog_peer {
    color: #e0dee2 !important;
    
}

.im_dialog:hover, .im_dialog_wrap:hover, .im_dialog_message_wrap:hover, .im_dialog_peer:hover {
    background-color: rgb(50,40,60) !important;    
}

.im_message_body, .im_dialog_badge {
    background-color: rgb(50,40,60) !important;    
    color: #e0dee2 !important;
}

.im_dialogs_scrollable_wrap a.im_dialog {
    border-bottom: 1px solid rgb(50,40,60);
    border-top: 1px solid rgb(50,40,60);
    margin-top: -1px;
    border-radius: 0 !important;
    padding-right: 11px;
}

.composer_rich_textarea, .im_message_field {
    background-color: rgb(20,10,30) !important;    
    color: #e0dee2 !important;
}
.im_record:hover{
    background-color: rgb(20,10,30) !important;    
}

.im_record {
    background-color: rgb(70,60,80) ;
    color: #e0dee2 !important;
}

.message_actions_wrap {
    width: 210px;
}

/* Created via Magic CSS extension for Chrome - https://chrome.google.com/webstore/detail/ifhikkcafabcgolfjegfcgloomalapol */