As mentioned in the CSS guide we encourage people to not make significant changes to built in UI as you can introduce incompatibilities for mobile devices, accessibility, in addition to resulting in breakage when we need to make updates.
That said, I added a few more CSS variables to the grid container so you can switch to using flexbox with centered items with something along the lines of:
.mode_grid .user_game_grid {
display: flex;
justify-content: center;
flex-wrap: wrap;
}
.mode_grid .user_game_grid .game_cell {
width: var(--itchio_grid_cell_width);
}
(Note that the page previously never used flexbox, but was using inline-block elements with some margin tricks)