I think that the problem was that you changed it in
MM5 > skin > skin_base.less
but most skins overrides it, so ensure that you also modify it in
MM5 > skins > YourSkin > skin_base.less

i.e. in case of the 'Material Design' it is
MM5 > skins > Material Design > skin > skin_base.less

Otherwise yes, skins like "Material Design" let you to set the font size in Options > Skin > Size: [Small | Normal | Large]

EDIT: While testing the font size settings in Options I have found an issue, to be fixed as https://www.ventismedia.com/mantis/view.php?id=19299 (details there)

Thank You for Your quick reply. That was not really what I want, but I've found a skin that works fine for me. Thank You
Baggins

some skins have font size [url=https://www.dropbox.com/s/o3n67s4y9th4q4n/some%20skins%20have%20font%20size%20flexibility%20baked%20in.png?dl=0]flexibility already baked in[/url]
So if you use one of those you may get all the adjustment that you need.

Hi!
I want to change the fontsize in MM5. Although I have no knowledge about css or less, I tried to change the values of @baseFontSize and @fontSizeMultiplier in MM5 > skin > skin_base.less. Depending of @fontSizeMultiplier the fontsize was doubled or trippled . It seems that the cahn ge of the original @baseFontSize (13px) has no effect. I need a fontsize of 140% or 150%. Can anyone help me?

Baggins