The LFDSSTS Login page is modifiable in that you have access to the web file and can modify the HTML sections, which are not particularly complicated. There is currently no GUI designer as for WebLink and Forms.

That file's default location is: C:\Program Files\Laserfiche\Directory Server\Web\WebSTS\Views\Home\Login.cshtml

You may also want to modify the similar "Embedded.cshtml" file in the same directory. This is the version of the LFDSSTS login page loaded in embedded windows with the Laserfiche Windows Client, Office Integration, etc.

Make sure to make a backup copy of the original Login.cshtml/Embedded.cshtml file before making any modifications. Note that installing LFDS(STS) updates will usually replace this file with a clean updated one, so you'll need to re-implement any modifications after LFDS upgrades.

Do not just add the whole modified file from the prior version back, as other parts of the file may have had important code changes with the update. 

Don't modify any of the C# function bits in the <script> section unless implementing the simple SAML IDP auto-selection customization described in this post: LFDSSTS auto login with AD FS

From a support perspective, we generally don't care if you modify the "look and feel" of the LFDSSTS login page and do it fairly often during implementations.