How can I completely remove a Theme from SharePoint?

To completely remove a BindTuning theme from your SharePoint environment (and not just uninstall it), you must follow the instructions below, depending on the SharePoint version you are using.

Modern SharePoint

To completely remove your Modern SharePoint theme, you'll necessarily need to run the provided PowerShell script. To do this:

Access your BindTuning account;
Navigate to your Design tab > My Themes;
Mouse over the installed theme and select More Details;
Click on Download;
After downloading your theme, extract the contents of the zipped folder;
Inside the folder, proceed to extract the underlying folder name THEMENAME.SPFx_x.x.xx.xx.zip;
Right-click on the file Installer.ps1 and select Run with PowerShell;
If prompted, download the necessary PnP PowerShell cmdlets by following the script's indication;
Select option 9 - Uninstall theme and full-color experience;
Paste the site collection URL from where you want to remove the theme from;
Select the uninstallation scope (remove from Site Collection App Catalog or from Tenant App Catalog).
If you're unaware of where the theme has been deployed, here you'll find detailed information relating to these same scopes.
The uninstallation process will proceed automatically.

Classic SharePoint

For Farm solutions (SP2013):

Info: SharePoint Designer is required in order to accomplish the steps described in this section. If you need to download it please follow this link.

Go to the sites where the theme was previously installed;
Now go to Settings > Site Settings > Manage Site Features and make sure the theme feature is deactivated;
Go to Site Settings > Masterpage, and make sure that the Seattle masterpage is applied;
On your server open the Central Administration;
Go to the solution gallery (/_admin/Solutions.aspx);
Check if the YourThemeName.SP2013.wsp is in this gallery;
If it is click in the solution, and retract it;
When the retract job is finished click on the solution and delete it;
On your server go to C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\TEMPLATE\LAYOUTS\1033\IMAGES
Delete the folder YourThemeNamePackage
Go to C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\TEMPLATE\LAYOUTS\1033\JS
Delete the folder YourThemeNamePackage
Go to C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\TEMPLATE\LAYOUTS\1033\STYLES
Delete the folder YourThemeNamePackage

For Farm solutions (SP2016 and SP2019):

Info: SharePoint Designer is required in order to accomplish the steps described in this section. If you need to download it please follow this link.

Go to the sites where the theme was previously installed
Now go to Settings > Site Settings > Manage Site Features and make sure the theme feature is deactivated
Go to Site Settings > Masterpage, and make sure that the Seattle masterpage is applied
On your server open the Central Administration
Go to the solution gallery (/_admin/Solutions.aspx)
Check if the YourThemeName.SP201X.wsp is in this gallery
If it is click in the solution, and retract it
When the retract job is finished click in the solution name again and delete it
On your server go to C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\TEMPLATE\LAYOUTS\1033\IMAGES
Delete the folder YourThemeNamePackage
Go to C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\TEMPLATE\LAYOUTS\1033\JS
Delete the folder YourThemeNamePackage
Go to C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\TEMPLATE\LAYOUTS\1033\STYLES
Delete the folder YourThemeNamePackage

For SandBox solutions

To completely uninstall a SandBox solution, we will need to remove the deployed masterpages, page layouts, and associated theme resources. To do this, follow the steps below:

Info: SharePoint Designer is required in order to accomplish the steps described in this section. If you need to download it please follow this link.

Removing Master Pages

Access your site collection using SharePoint Designer;
Go To All Files;
Open _catalogs > masterpage;
Search for a folder with the name of the theme and remove it.

Removing Page Layouts

Access your site collection using SharePoint Designer;
Go To All Files;
Open _catalogs > masterpage;
Scroll down, select all the .aspx files that start with ThemeNameTheme (e.g. SPCTheme), and remove them.