Sublime Text 3 how to change the font size of the file sidebar?

The answers are omitting the square brackets, in the case one is creating the file from scratch.

To recap, for the ST3 users who don't have the Default.sublime-theme file (which is actually the default configuration), the simplest procedure is:

1. Navigate to Sublime Text -> Preferences -> Browse Packages
2. Open the User directory
3. Create a file named Default.sublime-theme (if you're using the default theme, otherwise use the theme name, e.g. Material-Theme-Darker.sublime-theme) with the following content (modify font.size as required):

[
    {
        "class": "sidebar_label",
        "color": [0, 0, 0],
        "font.bold": false,
        "font.size": 12
    },
]

For reference, here there is the full file (as found in ST2).

Ubuntu 18.04

Location of theme setting on Ubuntu 18.04, installed via sudo apt install sublime-text:

~/.config/sublime-text-3/Packages/User/Default.sublime-theme

MacOS

Location of theme setting on MacOS, installed via DMG:

~/Library/Application\ Support/Sublime\ Text\ 3/Packages/User/Default.sublime-theme

On Ubuntu, for versions of Sublime older than 3.2, what worked for me was changing the dpi scale in Preferences > Settings — User by adding this line:

"dpi_scale": 1.10 

For Sublime 3.2, you can use the following line instead:

"ui_scale": 1.10

Adjust the scale value as needed. After this change, you have to restart Sublime Text for it to take effect.

Navigate to Sublime Text>Preferences>Browse Packages. You should see a file tree.

In the Packages folder, you should see

Theme - Default > Default.sublime-theme (substitute Default for your theme name)

Open that file and find the "class": "sidebar_label: entry and add "font.size".

example:

    {
        "class": "sidebar_label",
        "color": [0, 0, 0],
        "font.bold": false,
        "font.size": 14
    },

I followed these instructions but then found that the menu hover color was wrong.

I am using the Spacegray theme in Sublime 3 beta 3074. So to accomplish the sidebar font color change and also hover color change, on OSX, I created a new file ~/Library/"Application Support"/"Sublime Text 3"/Packages/User/Spacegray.sublime-theme

then added this code to it:

[
    {
        "class": "sidebar_label",
        "color": [192,197,203],
        "font.bold": false,
        "font.size": 15
    },
     {
        "class": "sidebar_label",
        "parents": [{"class": "tree_row","attributes": ["hover"]}],
        "color": [255,255,255] 
    },
]

It is possible to tweak many other settings for your theme if you can see the original default:

https://gist.github.com/nateflink/0355eee823b89fe7681e

I extracted this file from the sublime package zip file by installing the PackageResourceViewer following MattDMo's instructions (https://stackoverflow.com/users/1426065/mattdmo) here:

How to change default code snippets in Sublime Text 3?