NF12 Theme Customization — Stylesheet attachment not being loaded (v13)

[Jorge Edison Vallejo Merchán]

I'm trying to use the Theme Customization window (AD_UserDef_Theme) to override a theme's CSS via attachment, following the wiki example:
attachment:AD_UserDef_Theme/custom.css,<record-uuid>
Setup: iDempiere 13, base theme iceblue_c, record scoped to System/Org=*/Role=blank/User=blank, IsActive=Y, CSS file attached successfully (confirmed via AD_Attachment and the attachment dialog).
Questions:

Does the astyles request only get generated after full login (inside AdempiereWebUI/Desktop init), or should it also apply to the login screen itself?
Is the CSS from the Stylesheet attachment expected to be loaded additively (as an extra <link>/<style> after the base theme.css.dsp, so I can just override a few selectors) or as a full replacement?
Is there a known caching layer (MUserDefTheme.s_cache / m_fullMap) that requires a full server restart to pick up a new/edited record, beyond the usual "Reset Cache"?

Context: I initially hit a NullPointerException in AttachmentStyleSheetServlet because I referenced the AD_Attachment_UU instead of the record's own ID/UUID — fixed that per the source code, attachment now resolves fine via direct URL testing... but the browser Network tab shows no astyles request at all after login, even after a full service restart. Any pointers on what else could prevent the desktop from generating that request?