This CSS should get you started
.categories-and-latest {
flex-flow: row;
div.column {
flex: 1 1 75%;
&.categories {
flex: 0 1 25%;
}
}
}
Adjust the 25% until the categories column is your desired width. The other column will fill the remaining space.
To explain how the flex layout works a bit more… the shorthand is:
flex: grow shrink basis;
grow: a non-zero value allows the div to grow to fill the space available, 0 will prevent this behavior
shrink: a non-zero value allows the div to shrink to fill the space available, 0 will prevent this behavior
basis: essentially it’s the width (or height) the div starts off at before growing/shrinking