I've recently installed Xubuntu 18.04 on an old refurbished ThinkPad X250. It's working well, though one thing that would help usability a lot would be if I can increase the screen contrast. The blacks aren't especially dark, and the whites aren't especially bright - it's just not a very high-quality LCD - and it causes some eyestrain, especially when most applications are using lower-contrast colors that are intended to be readable on higher-contrast displays than this one.
To help things, I'd like to find a way to change display contrast settings and increase color contrast - if such a thing exists - or failing that use a post-process shader or similar tool to make dark colors darker, light colors lighter, and increase contrast that way. Prudent choice of xfce theme and application color schemes has helped a little, but global contrast adjustment tools would help much more.
I have already tried changing gamma settings. This makes the blacks darker, which is great, but it makes the whites darker too. So it doesn't really help anything.
I have also tried xcalib, and while it does have a contrast setting, contrast can only be decreased and cannot be increased using this tool.
Surely there must be options here. What can I do to make my screen easier to look at?