What you're seeing
The colors, background and spacing from your Custom CSS show up on the login page. Whatever your script was meant to do there doesn't: a line of text it adds, an image it swaps, an element it moves. No error appears on the screen.
Before you change anything, sign in and check whether the same script works on the pages after login. If it does, the script is fine and the login page is the only place it's missing. That narrows the problem to one screen.
What HighLevel documents for the login page
Agency custom code lives in Company settings, under Settings > Company > Whitelabel, in two boxes: Custom CSS and Custom JavaScript. HighLevel's help center documents customizing the login page with CSS. It doesn't promise that a script runs on that page.
Two login details don't need code at all. The agency logo set in the Whitelabel settings appears on the login screen, and the Whitelabel Domain decides which address serves the desktop login portal.
HighLevel also warns that custom CSS and JavaScript can break after interface updates. Unsupported code is something you recheck, not something you set once.
Why we keep scripts off the login
We've seen an agency login page serve the Custom CSS and leave the script out. Other notes from our builds describe a slot for the script on that same page. HighLevel's help pages don't say which to expect.
That uncertainty is the whole argument. A login built in CSS alone looks the same either way. A login that depends on a script rests on behavior nobody has documented, on the one screen every user of the agency sees first.
If the login screen needs it, it goes in Custom CSS.
Moving the login work into CSS
Much of the visual work a login script might do can be done in CSS: colors, backgrounds, images, spacing, and hiding an element you don't want.
- Open Settings > Company > Whitelabel. Set the agency logo there and check the Whitelabel Domain before writing any code for either.
- Sign out, open the login page on your Whitelabel Domain, and inspect the elements you want to change. Use the selectors you find today, not ones copied from an older template, because HighLevel's markup changes between versions.
- Write down each thing your script did on that page. Next to each one, write the CSS that does the same job.
- Put those rules in Custom CSS and save. Remove the login part of the script so nothing on the page depends on it.
- Load the login page in a private window and compare it with what you expect, field by field and image by image.
- Note the date. Check the login page again after HighLevel's next interface update.
Rather have it set up for you?
After a 30-minute 1:1 with Rob to look at your current login, we move its customization into CSS that needs no script and test it on your login domain.
The logo rule that matches nothing on the login
If you give a sub-account its own logo with CSS scoped to that sub-account's location id, the rule has nothing to match on the agency login page. In our builds, location-scoped logo rules never applied there, because the login sits at the agency level.
Style the login logo with a rule that doesn't depend on a location id, or leave it to the agency logo setting. Keep the location-scoped rules for the screens inside the sub-account they were written for.
Signed-in screens are a separate question
If the trouble is with navigation after sign-in rather than the login page, look at Custom Navigation. HighLevel gives it a separate control for reusing legacy agency code, so check that control on its own before you assume the code itself is wrong.