How to override !important?

Apart from overriding a style set by the style attribute, the !important should only be used when you have selectors in your style sheet with conflicting specificity.

But even when you have conflicting specificity, it is better to create a more specific selector for the exception. In your case it's better to have a class in your HTML which you can use to create a more specific selector which doesn't need the !important rule.

td.a-semantic-class-name { height: 100px; }

I personally never use !important in my style sheets. Remember that the C in CSS is for cascading. Using !important will break this.

Disclaimer: Avoid !important at all cost.

This is a dirty, dirty hack, but you can override an !important, without an !important, by using an (infinitely looping or very long lasting) animation on the property you're trying to override the importants on.

@keyframes forceYellow {
  from {
    background-color: yellow;
  }
  to {
    background-color: yellow;
  }
}

div {
  width: 100px;
  height: 100px;
  margin: 0 auto;
  background: red !important;
  animation: 1s linear infinite forceYellow;
}

<div></div>

Overriding the !important modifier

1. Simply add another CSS rule with !important, and give the selector a higher specificity (adding an additional tag, id or class to the selector)
2. add a CSS rule with the same selector at a later point than the existing one (in a tie, the last one defined wins).

Some examples with a higher specificity (first is highest/overrides, third is lowest):

table td    {height: 50px !important;}
.myTable td {height: 50px !important;}
#myTable td {height: 50px !important;}

Or add the same selector after the existing one:

td {height: 50px !important;}

Disclaimer:

It's almost never a good idea to use !important. This is bad engineering by the creators of the WordPress template. In viral fashion, it forces users of the template to add their own !important modifiers to override it, and it limits the options for overriding it via JavaScript.

But, it's useful to know how to override it, if you sometimes have to.