I listed some of the issues I've reported to support through this last 12 months, in case they are useful to supplement your pre-release checklists. Hope it helps.
Unthemed text background colour when pasting draft content from editor from one forum into another on Here comes Santa 2018.
Default unthemed IPS colours showing in following areas:
Unthemed areas in Calendar Reviews & Comments,
Top and bottom of the pop-up Notifications list unthemed (when you click the bell icon)
Chatbox Emoticon top and bottom bars unthemed
Unthemed CSS code, quote and spoiler box CSS
Difficult to see colours:
Geode - Reputation Points in postbit (forum) colour were hard to read when member has more than 0 points.
Spring Fling, Ivy League and Simplicity Red and themes - main menu was overlapping some buttons preventing user from clicking on Unread Content or any breadcrumb links.
Geode had a menu item glitch seemed to be a fixed CSS width too short for menu text
If you scroll down the calendar, a floating white tool bar appears in default colours.
I noticed in the CSS for Hallows Eve, there was a call for a png file at IBTheme.com which doesn't exist and causes a 404 on my site. .ipsApp select:not([multiple]) / select_dropdown.png
When using grid layout there was some left over code causing double category footers to appear. Fixed by Pete on every theme.
Nexa - iPad - Search box covering user menu.
Halloween Grid layout was broken when using an iPad in landscape mode, the header is much shorter than the rest of the page, either with or without Chatbox enabled.
Christmas Noel 2018 and Here Comes Santa header broken in iPad landscape mode (Pete fixing as I type!), but can't test White Christmas as not yet available in theme chooser menu.