Hi Marjolaine12 and lechicbynadia,
I am sorry if I wasn’t clear. You should not paste the style into your page. This will not work.
You would need to paste the code that follows using a custom CSS plugin to change the style. This is special code used to allow the appearance of an element on a web page.
Although your theme may have a customization option to add custom CSS, I use a plugin called Simple Custom CSS plugin. (https://www.ads-software.com/plugins/simple-custom-css/)
You should add this to your WordPress installation. The best way to do this is by clicking on “Plugins”, “Add New” and searching for “Simple Custom CSS Plugin”.
Once this plugin is installed, you will see a new option under “Appearance” for “Custom Css”. Choose this option, enter the below pasted code and click on the Save button.
.thumbnailblock {
width: 170px!important;
}
Let me know if this is not clear and I will try to find a more visual way to represent this for you.