You can force apply CSS using 3 methods.
1) Create a token and place it at the bottom of your page template. Then you can input the CSS into the token and it should be appled instead of the marketo default as its read last.
2) You can go to the form and there is an option to insert custom CSS. (edit form > Form settings > click cog > custom CSS)
3) Place a HTML snippet on the page and then insert the CSS into it.
The problem you are having might have something to do with the width of the form not displaing properly.
If you use the custom CSS option insert this:
.mktoForm fieldset {
width: 600px !important;
}
or try inserting a HTML snippert and in it put the following:
<style>
.mktoForm fieldset {
width: 600px !important;
}
</style>
Simply change the width to the correct amount needed e.g. 500px or 350px
The !important means it will override any other CSS on the page trying to do the same thing. I have had issues where pages being displayed on an overlay wouldnt display forms properly and this code help with the issue. However you can use this technique to change pretty much anything about the form as long as you can find out the class name of each element.
For example sometimes I might want to move where the tick box is. You can do this by right clicking the tick box in a google chrome window (on an approved version of the page not in the edit screen) and then click inspect element. You should then be able to find its class / id name which will have a unique number. Once you know that you can change it however you like using CSS.
In the above case that is a standard class name though which is much easier to find as its a common class name used throughout the form.