Installation is easy, follow the steps below and it will be fully installed.

1. Create a new Google Sheet or Document.

2. Go to extensions/apps script

3. Select and make a new project

4. On the left, and click the plus button above "library."

5. Paste the following ID in the box and press look up.

1LcDmG8gFKv8HQ3VwovO0ifyHS7TWjBMup6kfPhKBTOkJ1pBNE3eDgiKm

6. Select the version of your choice and add it.

7. Success! BetterGUI is fully installed.

[BetterGUI is still in progress]
[NOTE THAT BETTERGUI WORKS BOTH FOR GOOGLE SHEETS AND DOCS]

Since BetterGUI offers UI content, there are a few things that you can do with it.

Lets start off with the simple.

You can launch the UI by writing the code below:

BetterGUI.popUp("Title goes here", "Text goes here");

Upon running the function, you will get a message similar to this:

You can also use BetterGUI to display input, such as the following:

function getText() {
  var input = SpreadsheetApp.getActiveSheet().getRange('A1').value; //Retrieves text from cell A1
  var title = SpreadsheetApp.getActiveSheet().getRange('A2').value; //Makes the title the value of A2
  BetterGUI.popUp(input, title);
}

BetterUI has simplified the HTML process.
Here is an example of how to use:

function showDialog() {
  var html = ''; //You can add html here
  BetterGUI.showDialogFromHtml(html, 'title'); //Removes a whole lot of steps
}

//You can do the same with sidebars:
function showSidebar() {
  var html = ''; //Yet again add html
  BetterGUI.showSidebarFromHtml(html, 'title'); //Works the same way as dialog but with sidebar
}

BetterGUI has been made open source under NO licensing (it's absolutely free to use and edit) so that others can fit their parts into it to make it a better program.

You can access the open source code here