how to overwrite css style

I'm developing pages, now in my css style I have this line of code

.flex-control-thumbs li {
    width: 25%;
    float: left;
    margin: 0;
}

for my pages. Now, some of my pages don't need this line

width: 25%;
float: left;

It is possible that I can overwrite it in internal css of the page, which will cause the original behaviour to be ignored?

Using !important is not recommended but in this situation I think you should -

Write this in your internal CSS -

.flex-control-thumbs li {
  width: auto !important;
  float: none !important;
}

instead of overwriting, create it as different css and call it in your element as other css(multiple css).
Something like:

.flex-control-thumbs li 
{ margin: 0; }

---

Internal CSS:

.additional li
{width: 25%; float: left;}

---

<ul class="flex-control-thumbs additional"> </ul> /* assuming parent is ul */

You can create one more class naming

.flex-control-thumbs-without-width li {
width: auto;
float: initial; or none
}

Add this class whenever you need to override like below,

<li class="flex-control-thumbs flex-control-thumbs-without-width"> </li>

And do remove whenever you don't need for other <li>

Just add

.flex-control-thumbs li {
width: auto; 
}

Yes, you can indeed. There are three ways of achieving this that I can think of.

1. Add inline styles to the elements.
2. create and append a new <style> element, and add the text to override this style to it.
3. Modify the css rule itself.

Notes:

1. is somewhat messy and adds to the parsing the browser needs to do to render.
2. perhaps my favourite method
3. Not cross-browser, some browsers like it done one way, others a different way, while the remainder just baulk at the idea.

Increase your CSS Specificity

Example:

.parent-class .flex-control-thumbs li {
  width: auto;
  float: none;
}

Demo:

.sample-class {
  height: 50px;
  width: 50px;
  background: red;
}

.inner-page .sample-class {
  background: green;
}

<div>
  <div class="sample-class"></div>
</div>

<div class="inner-page">
  <div class="sample-class"></div>
</div>

You can add your styles in the required page after the external style sheet so they'll cascade and overwrite the first set of rules.

<link rel="stylesheet" href="allpages.css">
<style>
.flex-control-thumbs li {
  width: auto;
  float: none;
}
</style>