In terms of the second request you will need the base files or to get the necessary files from an existing skin already out there made by someone else although this could be edited significantly. There's guides on extracting base files and using the resource archiver in the stickies. If you don't know what I'm talking about you're going to have to put in some time reading and learning, non-negotiable I'm afraid.
The above is the directory you need in order to find the highlighting folders and graphics from the base files (if you choose this option) so it's the sitoolkit you'll need to extract (I do so to my desktop). Within the squad folder there is several folders labeled so you will know exactly what each highlighting folder relates to. Just change the colour in the row.xml to your preference, i.e. <colour id="red_replacement" name="squad_outside"/> change that in bold to an actual colour's name like "black".
All edited graphics then need to go into the equivalent position (I've bolded this part of the directory to highlight the equivalent location I'm referring to show the similarly of the extracted location with the final location) within your skin, i.e. C:\Users\YOUR NAME\Documents\Sports Interactive\Football Manager 2018\skins\YOUR SKIN'S NAME\graphics\tables\custom\squad
In terms of the first question you may need to extract the sidebar menu table.xml. So back to extracting the base files. This will be contained in the panels folder I imagine (was a while ago since I extracted it). This panel is then found in the generic folder within.
I would then look to experiment changing any text sizes contained in the panel as I'm not definitely sure on the exact code without investigating. To find the sizes within do a search (I use notepad++ free software to help with editing code and this allows you to search within the code too, i.e. ctrl+F and then look for 'size'). This will point you to all the size options. Increase any numbered sizes or if it says 'small', go for 'large', etc. Make sure to leave the panel open so you can always undo anything which doesn't effect the size of the writing. Just a bit of trial and error.
As I say I'm less sure on the second option. I know the action-bar (the secondary bar at the top) is edited via an xml in the classes folder and as I was writing part of me was wondering if this is the case here (again this folder is extracted in the same way as the other folders I've suggested, just instead you're looking for 'classes'). So if you don't have joy with the paragraph directly above, keep an eye out for any container or widget classes stated in the sidebar menu table.xml, i.e. <container class="tab_menu_strip".... as a completely unrelated example. If you see any classes mentioned it might be worth going this route.
Clearly before anything else though you need to extract the base files so getting familiar with this and reading the stickies is important if this is new to you.
NB: I'm assuming the files aren't already present in your skin so this is worst case scenario, search your skin's folders to see if you actually have any of these files first. I didn't want to assume you had anything.