I know very little about theme'ing and such, so cannot really be of much help sorry. I tried before fixing a field hard to read in win32 theme, but didn't even succede in that lol
That file you're looking at, is only working on gtk2, and this is gtk3 here in deluge2. The two built-in themes, adwaita and win32, doesn't have any files to edit in the installation, as built into gtk. I can supply you a zip of the folder/files needed from git, so you can take a look yourself and see if you can make changes you want, if you tell me if you want adwaita or win32 or both. Then if adding such folder with same name as theme and edited theme files in it, then those changed files will be loaded when said theme is defined, i.e. a win32 or adwaita folder added under 'data\share\themes'. I used to do that previously to add a bugfix from git to win32 theme, but is included now so no need anymore.
We are at gtk 3.24 currently, and before gtk 3.10 there where nice options for setting toolbar-size and other stuff, but got removed in later releases, and then was solely up to the devs to configure such themselves. There is an option not removed for setting font and size, in 'data\etc\gtk3.0\settings.ini, named 'gtk-font-name', also 'gtk-font-size', but you can also set size from the first option by using both name and size. You don't need double-quotes in gtk3 for values with spaces in between btw. Howeer, in my testings, I couldn't change font and size from those settings. I'm guessing that they only work when using a theme which doesn't already state which font and size to use, which probably overrides the settings.ini, since it didn't work in my tests, or else i'm just doing something wrong though, but am simply following gtk3 reference manual, and various examples of values online for other gtk3 apps. There where suggested e.g. 'gtk-font-name=Arial 14' but didn't change anything. The default value is 'Sans 10' for that value in gtk3. The win32 theme uses in there CSS for font and size I could see: 'Segoe UI 9', so as said, probably overrides settings.ini with app specific value, as settings.ini is for general values for all gtk apps supporting it.
The toolbar size and button size also is now only defined in those CSS files, I believe as setting min width/height of buttons or toolbar, and 16/16 is small and 24/24 is normal(big), but again I had a hard time figuring it out, though granted only spent little time on it.
If you wanna see first if you think you will be able to change stuff then here are some links to said CSS files so you can see the syntax:
win32 theme files:
https://github.com/GNOME/gtk/tree/gtk-3 ... heme/win32
Adwaita theme files:
More than this, but main file I believe:
https://github.com/GNOME/gtk/blob/gtk-3 ... tained.css (gtk.css just calls this one and nothing else, but there are other files called from this though).
Toolbar-button changes usually on gtk3 apps needs you to download a gtk3 icon theme, and place folder of that under 'data\share\icons' and add the new theme under 'data\etc\gtk-3.0\settings.ini' as 'gtk-icon-theme-name=xxxxxx', where xxxxx is name of folder added before with the icon theme. Never messed with that myself, especially as so few buttons. Atleast Cas stated many years ago that it was default gtk icons, and should install new icon theme if wanted it changed, but I don't know if things have changed since.Actually I just skimmed through the UI code and it seems deluge hardcodes some icon names, and they aren't included(there are some included, like the droplet, flags and icons from lowest small bar etc, but not any from the main toolbar) and looking the names up suggest they are from gnome icon set. Cas develops on ubuntu I believe, which maybe uses gnome icon set I dunno, but would guess it. Anyway, I don't really have dwelled into that, so don't really know honestly much about it. By default adwaita icon set is used when not having defined an icon set.
Sorry couldn't help you with this unfortunetly.
I will mess little around with the win32 theme when time/motivation for it, as e.g. still wan't that almost unreadable line fixed(white text on light-grey background), and then at same time can look at if can make toolbar bigger, and as said the font already is defined there as Segoe UI 9, so I guess a simple changing of that value(9) would up the font size seemingly(and font name/type changed if wanted). I'll post if find something out, but will probably not be that quickly honestly.
Edit: OK, I fixed the issue of hard to read unfocused selected items, which was an issue in win32 theme, but not in adwaita and other themes, so will upload new fixed installers probably sometime tomorrow.
@PhilliP, if you use win32 theme as per default in these installers, then when I upload new installers, you can change font size or/and type in file 'data\share\themes\win32x\gtk-3.0\gtk-win32-base.css' on line 25 and 26, for type and size, e.g. change line 26 of "font-size: 9pt;" to instead "font-size: 11pt;". Still haven't found out about how make toolbar bigger yet though. Well, I can make the bar bigger, but not the icons themselves yet, so there is just extra empty padding over and under, which doesn't help readability. However, in adwaita theme by default where the bar is bigger, then the icons are still the same small size there, so i'm thinking there isn't bigger versions of these used icons included in gtk as else they would be bigger too in adwaita's bigger bar. So another icon theme would be needed presumably, or so i'm guessing atleast.
Edit2: BTW, just found out that gnome-icon-theme is actually adwaita-icon-theme, just got renamed 5 years ago into adwaita-icon-theme, which is default in gtk3, so installing new gtk icon theme should work I believe, if wanted. I could also find out which specific files are used for main toolbar and you could try replace them under there folder, but probably easier simply use alternative gtk icon theme instead.