If the tool is no longer available, you can use any editor and simply do a search and replace within the child theme style.css file, just as you would in something like a Word document. Brackets is a free, open-source code editor that is recommended and works really well with Windows and macOS. You can get it here:

You will most likely want to change the colours to match your brand. All of the primary colour settings are within the stylesheet for easy swap out. You can find the colour codes in the comments at the top of the stylesheet.

To change your colours you can use this handy tool

Before making any edits to the theme, open the webpage linked to above and click on the ‘Choose file’ button, select the style.css file from the download package and then click on the ‘Load Data’ button.

You will then see this screen. Select your new colours to replace the existing colours and then click the ‘Generate’ button.

You will be given a new stylesheet with your own colours to download. Click the ‘Open CSS File’ button and copy the entire contents of that page to your clipboard.

You can then navigate to Appearance>Editor and in the style.css file replace the contents with the contents of your clipboard and save. Your site will now be in your chosen colours.

Alternatively you can right click and save the file and then upload it to your site via FTP.