This is somewhat overdue complaint about XC developed themes.
XC5 comes with few ready STOCK themes. They are kept up to date with every release of XC.
There are also many NON-STOCK themes developed by XC team. I had few problems with these themes not being kept up to date with XC releases. This is latest one
https://market.x-cart.com/ecommerce-templates/multistore.html
Author: QT
I would expect these themes developed by XC/QT to be kept up to date since it is all internal - theme development can be easily matched with the XC releases - but they are not. It seem once released they just sit there until someone reports an issue with them.
The above mentioned theme is 5.3.x ready. It was installed probably around 5.3.3.1 (or 2) and working. Now we are at 5.3.3.4 and the theme miserably fails at checkout page. Page loads with errors and no styling. Now this is dev store so no harm done but imagine live store where the owner has little to no understanding what/how to do upgrades and just uses the promised "one click update" - bam no orders coming in anymore...
The reason with this right now - the theme overrides skins/customer/css/less/variables.less file and the required css variables for the newest checkout page no longer exist....
They are included with the stock file and stock themes work without problem. But the non-stock theme does not have them.
So this again brings in the question of XC upgrades - the one discussed so many times and with so many complaints how the XC4 upgrades are so difficult with all the 3rd party modules.... Well guess what - the XC5 upgrades are starting to prove no less difficult if not actually more difficult than XC4 ones.