Synology Notes UI update using TamperMonkey, install Tampermonkely extension in your browser, then click the Raw button on this page to install the script

Synology_Notes_UI.user.js

// ==UserScript==
// @name         Synology Notes UI
// @homepage     https://github.com/justinkelly/Synology-Notes-UI-update-tampermonkey
// @version      0.2.8
// @description  Make Synology Notes UI look similar to Notion
// @author       Justin Kelly
// @include      https://*/?launchApp=SYNO.SDS.NoteStation.Application*
// @icon         https://global.download.synology.com/download/Package/img/NoteStation/2.6.1-2450/thumb_256.png
// @namespace    https://github.com/justinkelly/Synology-Notes-UI-update-tampermonkey
// @supportURL   https://github.com/justinkelly/Synology-Notes-UI-update-tampermonkey
// @downloadURL	 https://gist.github.com/justinkelly/95e7ea152be9ffc149cfd435b4714d65/raw/Synology_Notes_UI.user.js
// @updateURL	https://gist.github.com/justinkelly/95e7ea152be9ffc149cfd435b4714d65/raw/Synology_Notes_UI.user.js
// @grant        none
// ==/UserScript==


function GM_addStyle(css) {
  const style = document.getElementById("GM_addStyleBy8626") || (function() {
    const style = document.createElement('style');
    style.type = 'text/css';
    style.id = "GM_addStyleBy8626";
    document.head.appendChild(style);
    return style;
  })();
  const sheet = style.sheet;
  sheet.insertRule(css, (sheet.rules || sheet.cssRules || []).length);
}

//body
GM_addStyle("body {font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Oxygen', 'Ubuntu', 'Cantarell', 'Fira Sans', 'Droid Sans', 'Helvetica Neue', sans-serif';}");
GM_addStyle(".syno-ns-win .syno-ns-note-snippet-container .syno-ns-note-snippet-title { font-weight: 600;}");

GM_addStyle(".sds-window-v5 .x-layout-split.x-layout-split-west {border-right-width: 0px;}");
GM_addStyle(".syno-ns-note-container-panel {border-right-width: 1px;border-right-color: rgb(223, 225, 228);}");
GM_addStyle(".x-layout-split-west {background: rgba(55, 53, 47, 0.08)}");

//remove snippet/date in list
GM_addStyle(".syno-ns-note-snippet-thumbnail{display:none;}");

//top menu bar
GM_addStyle(".syno-ns-win .x-window-tl {background: rgb(244, 245, 247)}");
GM_addStyle(".sds-standalone-main-window-header-text {display:none}");
GM_addStyle("#ext-gen60 {display:none}");
GM_addStyle("#ext-gen65 {z-index: 2; overflow: initial;}");
GM_addStyle(".sds-standalone-main-window .sds-standalone-main-window-header { padding: 7px 18px;}");

GM_addStyle("#ext-gen65 {float:right}");
GM_addStyle(".syno-ns-win .syno-ns-mainpanel .syno-ns-toolbar {padding-top: 8px;}");
GM_addStyle("#ext-comp-1230 {background-color: rgb(244, 245, 247); border-bottom: 1px solid rgb(223, 225, 228);}");
GM_addStyle(".sds-window-v5 .x-panel-tbar .x-toolbar-layout-ct { padding-bottom: 4px;}");
GM_addStyle(".syno-ns-win .syno-ns-mainpanel .syno-ns-toolbar { padding-top: 4px;}");
GM_addStyle("#ext-comp-1233 { padding-top: 0px;}");




//left panel
GM_addStyle("#ext-gen96 {border-right: 1px solid rgb(223, 225, 228)}");

//middle panel
GM_addStyle(".syno-ns-win .syno-ns-note-container-panel {padding-left: 0px;padding-top:5px;}");
GM_addStyle(".syno-ns-win .syno-ns-note-snippet-container .syno-ns-note-snippet-content {  color: rgb(46, 46, 46);   opacity: .5;}");

//search bar
GM_addStyle("#ext-gen79 {background:#f3f2f1}");

//tree background
GM_addStyle("div.syno-ux-treepanel {background: rgb(238, 239, 241);}");
//tree border right
GM_addStyle("#ext-comp-1022 {border-right: 1px solid #edebe9;}");
//tree icons
GM_addStyle(".syno-ns-win .x-tree-node .x-tree-node-el .x-tree-node-icon { opacity: 0.7; }");

//tree text
GM_addStyle(".syno-ux-treepanel .x-tree-root-ct .x-tree-node-ct a span {color: rgba(25, 23, 17, 0.6);font-weight: 500; }");

//tree text selected
GM_addStyle(".syno-ns-win .x-tree-node .x-tree-selected .syno-ns-notebook-title {font-weight: 600; color: rgb(55, 53, 47);}");
GM_addStyle(".syno-ns-win .x-tree-node .x-tree-selected .syno-ns-notebook-total {font-weight: 600; color: rgb(55, 53, 47);}");
GM_addStyle(".syno-ns-win .x-tree-node .x-tree-selected .x-tree-node-icon {background-position: 0px 0px;}");
GM_addStyle("div.x-tree-selected {color: rgb(253, 253, 253);")
GM_addStyle(".syno-ns-win .x-tree-node .x-tree-selected {background: rgb(253, 253, 253);");


//main white section
GM_addStyle("#ext-comp-1239 { padding-top: 0px;}");
GM_addStyle("#ext-comp-1022-xsplit {border-top: 1px solid #edebe9;}");

//middle panel
GM_addStyle("#ext-gen135 {padding-left: 7px;}");
GM_addStyle("#ext-comp-1028-xsplit {display:none;}");
GM_addStyle(".syno-ns-win .syno-ns-note-snippet-container {margin: 0;padding: 8px 2px 0px 5px;border-color: rgb(223, 225, 228);}");
GM_addStyle(".syno-ns-win .syno-ns-note-snippet-group-container.syno-ns-note-snippet-container-select .syno-ns-note-snippet-container { background-color: rgb(244, 245, 247); border-left: 2px solid rgb(8, 109, 214); }");
GM_addStyle(".syno-ns-win .syno-ns-note-toolbar-ct {  margin-right: 0;  border-color: rgb(223, 225, 228); }");
GM_addStyle(".syno-ns-win .syno-ns-note-group-header {  background-color: rgb(238, 239, 241);    color: rgba(25, 23, 17, 0.6); font-weight: 500;  padding: 0px 10px; border-color: rgb(223, 225, 228);  margin-right: 0px;}");

//buttons
GM_addStyle(".syno-ux-button-default { background-color: inherit;}");
GM_addStyle(".syno-ns-win .syno-ns-mainpanel .syno-ns-toolbar .syno-ns-create-btn {background-image: none; background-color: rgb(8, 109, 214); border: 1px solid rgb(8, 109, 214);}");
GM_addStyle(".syno-ns-win .syno-ns-mainpanel .syno-ns-toolbar .syno-ns-create-btn:hover {background-image: none; --tw-brightness: brightness(1.25);filter: var(--tw-blur) var(--tw-brightness) var(--tw-contrast) var(--tw-grayscale) var(--tw-hue-rotate) var(--tw-invert) var(--tw-saturate) var(--tw-sepia) var(--tw-drop-shadow);}");
GM_addStyle(".syno-ns-win .syno-ns-mainpanel .syno-ns-toolbar .syno-ns-create-btn.syno-ux-button-dropdown.syno-ux-button-split button:first-child { border-right-color:  rgb(8, 109, 214);}");

//3rd level notes
GM_addStyle("[aria-level='3'] .syno-ns-notebook-fullmenu-notebook-common-icon { margin-left:10px;}");

//editor
GM_addStyle(".synohdpack .syno-ns-win .syno-ns-editor-normal-panel .mce-container .mce-container-body .mce-toolbar-grp {border-bottom: 1px solid rgb(223, 225, 228);padding:0;background-image: none;}");
GM_addStyle(".syno-ns-win .syno-ns-editor-normal-panel { padding: 5px 0px 0px 4px;}");
GM_addStyle("#ext-comp-1053 table.x-toolbar-ct { color: rgb(55, 53, 47); opacity: 0.5;}");

/* Colours

    --themeDarker: #004578;
    --themeDark: #005A9E;
    --themeDarkAlt: #106EBE;
    --themePrimary: #0078d4;
    --themeSecondary: #2B88D8;
    --themeTertiary: #71AFE5;
    --themeLight: #C7E0F4;
    --themeLighter: #DEECF9;
    --themeLighterAlt: #EFF6FC;

    --neutralDark: #201f1e;
    --neutralPrimary: #323130;
    --neutralPrimaryAlt: #3b3a39;
    --neutralSecondary: #605e5c;
    --neutralTertiary: #a19f9d;
    --neutralTertiaryAlt: #c8c6c4;
    --neutralQuaternary: #d2d0ce;
    --neutralQuaternaryAlt: #e1dfdd;
    --neutralLight: #edebe9;
    --neutralLighter: #f3f2f1;
    --neutralLighterAlt: #faf9f8;
    */