I don't know if custom themes are still a supported use case scenario, however when porting an old Windows 7 theme I made to the latest version of SAB I ran into two issues that I can't seem to figure out.
1) The programs list seems to double-render under the program's label. For example in the screenshot below I used the included Windows 7 theme and modified the StartPanelComposited::StartPanelPriv, Part# 6, State# 0 with a solid color #3C000000. As you can see the the individual list items are black as a result.
2) The second issue is, SAB seems to no longer take into account the DEFAULTPANESIZE:RECT value in StartPanelPriv, Parts# 6, 7, 9, 10, 14, 15, 17, 18, 19, 20, and 22
The older StartIsBack used to work fine in this regard (see: https://www.deviantart.com/longhornfusion/art/M7-with-new-Start-Menu-380790291 (although this is from 10 years ago))
Is there anyway to work around these?