Edit:

Since I pushed this out very quickly due to the big problem in 2025.1.0, some changes that weren't fully tested slipped in too.
Some advanced stylings may break.

One of the major changes is how styles are applied to cards.
Previously, styles and classes were applied to the ha-card element if it existed. Now they are applied on the new hui-card element, which is guaranteed to exist.

That means you need to update things like:

card_mod:
  class: my-class
  style: |
    ha-card.my-class { color: red; }

to:

card_mod:
  class: my-class
  style: |
    :host(.my-class) ha-card { color: red; }

Original release notes

• Fix broken styling of entity card rows in Home Assistant 2025.1
• Improve styling of new ha-card and ha-badge elements
  • This means stack cards can now be styled, and cards in Sections dashboards should work better
  • There is now no reason whatsoever to use mod-card