Tom,
You are using the Light Blue theme if I´m not mistaken and there has been some changes to that theme in the latest version. The font sizes etc. is one of them.
Regarding best solution, changing the font etc. in the theme files / CSS is possible but PROBABLY not an easy task. There are many different CSS classes you have to change and also some of the CSS classes are interconnected meaning if you change the font style for a column header, you might get a change somewhere else you don´t want. I don´t think it´s possible to just change a CSS class or two to go back to the way it was/worked before. The easiest way / solution is to change the width of your columns instead so they fit/work in your app.
If you are interested in a custom theme, I do offer consulting services where I can do that for you and would basically mean I create a custom theme for you. If interested, send an email to: info@awareim-consulting.com .