Hi - I had the same issue with background images. You will find the client object browser here:
driveletter:\Users\yourname\Documents\Sports Interactive\Football Manager 2024\skins\FM24 Base Skin (Transparent)\panels\client_object
You then need to increase the transparency number from '0' to your own preference. I went with '0.85'
<widget class="inner_contrast_box" appearance="boxes/custom/interface/content/secondary/paper" transparency="0.85">
The only annoying thing is that some of the sections (e.g. transfers, club info) are not transparent for me. Not sure if anyone knows how to change these?! Have attached an examples of it working and not.