Currently, the checkboxes are not styled under the base obsidian theme.
The following code might help:
input.task-list-item-checkbox {
border: 1px solid var(--background-modifier-border);
appearance: none;
-webkit-appearance: none;
}
input.task-list-item-checkbox:checked {
background-color: var(--background-modifier-border);
box-shadow: inset 0 0 0 2px var(--background-primary);
}