How to apply themes?

I see Themes in Settings and marking a listed theme as active, but how do you apply the theme to the company? I would like to make the pages easier for me to read, which I find hard to do with the default theme. The gray text, like for the inactive themes, is hard for me to read.

Theme are selected on individual transactions from among active themes.

Each individual transaction?

You can set up a default theme with Form Defaults
Read about form defaults: Set form defaults | Manager