The majority of FOEX components are styled using the Sencha Ext JS theme. This theme requires the Sencha CMD build tool to generate the theme after changes have been made, which is somewhat complicated for FOEX users like yourself since you rarely or never have to deal with Ext JS.
For a simple no hassle complete solution you can use our theme customization service: https://www.tryfoexnow.com/en/foex/custom-themes
Or if you want to avoid costs and try this on your own, you can devise your own CSS overrides by inspecting elements on the page using developer tools and working out the CSS rules to override them. This however may require adjustments on your side when we upgrade the underlying Ext JS framework in a future version e.g.
You will also need to copy and make adjustments to the triton APEX theme style.