Use responsive-friendly custom flex classes to build nested layouts with fixed and fluid columns.
The classes are named using the format flex-{property} for xs and flex-{breakpoint}-{property}
for sm, md, lg, xl, and xxl.
Where property is one of:
root - to define the root node of the flex layout(usually it's used when the layout is placed as a direct child of the body element in order to fully occupy the window height)
column-auto - to set a flex item with auto height
column-fluid - to set a flex item with fluid height
row-auto - to set a flex item with auto width
row-fluid - to set a flex item with fluid width
center - to set a flex item's content vertically and horizontally centered
start - to set a flex item's content alignments to start
end - to set a flex item's content alignments to end