One common theme is the struggle for personal identity. In such a novel, characters often feel lost within the marriage as there is no emotional bond. They might try to find themselves outside of the marriage, like through a new hobby or career. Another theme could be the impact on family and friends. Their unhappiness can spread and affect those around them, causing various relationship dynamics to shift.
Power dynamics can be a theme too. In a loveless marriage, one partner may try to dominate the other to fill the emotional void. Or perhaps, they compete for control over the relationship. Social expectations are also a common theme, as they often play a role in keeping the characters trapped in the loveless marriage, fearing the judgment of society if they were to leave.
The theme of loneliness is quite prevalent. Without love in the marriage, the characters experience a deep sense of isolation even while being with their partner. Also, there may be a theme of regret. They might regret getting into the marriage in the first place and constantly wonder about what could have been if they had chosen differently.