CSS Equivalent of the "if" statement

The @supports rule (92% browser support July 2017) rule can be used for conditional logic on css properties:

@supports (display: -webkit-box) {
    .for_older_webkit_browser { display: -webkit-box }
}

@supports not (display: -webkit-box) {
    .newer_browsers { display: flex } 
}

No. But can you give an example what you have in mind? What condition do you want to check?

Maybe Sass or Compass are interesting for you.

Quote from Sass:

Sass makes CSS fun again. Sass is CSS, plus nested rules, variables, mixins, and more, all in a concise, readable syntax.

CSS itself doesn't have conditional statements, but here's a hack involving custom properties (a.k.a. "css variables").

In this trivial example, you want to apply a padding based on a certain condition—like an "if" statement.

:root   { --is-big: 0; }

.is-big { --is-big: 1; }

.block {
  padding: calc(
    4rem * var(--is-big) +
    1rem * (1 - var(--is-big))
  );
}

So any .block that's an .is-big or that's a descendant of one will have a padding of 4rem, while all other blocks will only have 1rem. Now I call this a "trivial" example because it can be done without the hack.

.block {
  padding: 1rem;
}

.is-big .block,
.block.is-big {
  padding: 4rem;
}

But I will leave its applications to your imagination.

I'd say the closest thing to "IF" in CSS are media queries, such as those you can use for responsive design. With media queries, you're saying things like, "If the screen is between 440px and 660px wide, do this". Read more about media queries here: http://www.w3schools.com/cssref/css3_pr_mediaquery.asp, and here's an example of how they look:

@media screen and (max-width: 300px) {
  body {
     background-color: lightblue;
  }
}

That's pretty much the extent of "IF" within CSS, except to move over to SASS/SCSS (as mentioned above).

I think your best bet is to change your classes / IDs within the scripting language, and then treat each of the class/ID options in your CSS. For instance, in PHP, it might be something like:

<?php
  if( A > B ){
echo '<div class="option-a">';
} 
    else{
echo '<div class="option-b">';
}
?>

Then your CSS can be like

.option-a {
background-color:red;
}
.option-b {
background-color:blue;
}