kvantum is very closed, allowing only that it uses its own inbuilt themes.
I recommend using qt5ct, instead.
This will allow QT apps to use any system theme you use.
I highly recommend installing preload
sudo apt install preload
to allow much faster launching of QT apps.
Below is the Default .profile contents
if [ -n "$BASH_VERSION" ]; then
# include .bashrc if it exists
if [ -f "$HOME/.bashrc" ]; then
. "$HOME/.bashrc"
fi
fi
# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
PATH="$HOME/bin:$PATH"
fi
# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/.local/bin" ] ; then
PATH="$HOME/.local/bin:$PATH"
fi