The first function is meanwhile 2 years old, it might be outdated.
The other advice is a bit newer, but also outdated.
When Layout is active, lot's of the settings of the Theme will be removed/neglected, as you now use the Layout Plugin.
As example, see this thread:
https://toolset.com/forums/topic/definitive-method-to-center-horizontal-menu-in-toolset-starter-with-layouts/
Now, Toolset Starter comes with some CSS inbuilt, and we do not reccomend that theme anymore, exactly becuase we provide now full integration with all themes and this Starter Theme made many issues to clients, hence we do not develop it anymore in future.
We will solved BUGS that break sites, but we will not add any new feature.
I would reccomend, if you want a clean fresh theme, to strat on your own framework, which is pretty simple to achieve.
Here is a Blueprint:
hidden link
Use it as you like.
Related to wp-content\plugins\layouts\resources\css\cell-menu-css.css, that is not from the Theme, but from Layouts.
It is the Menu Cell. That is not customizable, unless with what you add to the actual Menu Cell settings.
You could also construct your own Cell, if you cannot work with the Bootstrap Menu that this cell provides:
https://toolset.com/documentation/user-guides/creating-custom-cells-unique-functionality/
Please let me know if with above information you can work further.