Update - Release 3
New features:
* Click directly on the live preview area to select an element for editing. (Middle click to toggle selection view now.)
* Undo-Redo system with 1000 states.
* Move swatch selection with arrow keys. (So you can keep the mouse over the color picker while changing swatches.)
* Gradient tool, lets you make a gradient between any two colors for use as a custom color picker.
* Custom color swatches, for temporarily storing relevant colors separately from the theme swatch list.
* Fullscreen mode.
* Buttons and edit box use dark colors to better fit the overall UI scheme, and reduce unnecessary contrast.
* Other UI details refined: e.g. Each group of buttons now have a unified highlight color.
* Added faint white grid lines to pickers for visibility in dark regions.
* Added a console to aid with diagnostics when troubleshooting, and to provide special features accessed via commands.
* Fix: Preview image no longer saves with gray flecks at the corners.
Downloads
(see first post for latest release)
(update: "C2 themes - Fisholith bundle v1" is now included.)
(click to enlarge)
Gradient picker
Right click on the left or right side of the gradient to set the current color as an edge-color for the gradient.
Likewise while performing any color picking action you can hold "<" or ">" keys or "A" or "S" keys to set the left and right gradient colors. By any color picking action I mean anything action that sets the color of a swatch, so a left-click in a color picker or a right-click anywhere on the screen will both work. (e.g. If you right click a color in the preview area while holding the "A" key, it will be set as the left edge color for the gradient.)
Console
Open the console with the F5 key.
Enter a command name, followed by a comma and arguments if any.
Commands are listed below.
showDisplayStats
Shows info about the monitor and Device Pixel Ratio (DPR).
scroll
Toggles free camera scrolling over the UI for monitor accessibility issues. Use {U,H,J,K} keys to move camera.
scale , <scaleFactor>
Scales the entire UI by the given scale factor.
e.g. "scale , 2" will scale the UI up by 2x.
unscale , <inverseScaleFactor>
Scales the entire UI by 1 divided by the given scale factor.
This is handy if you know your computer is scaling up by a factor, and you want to undo that scaling.
e.g. "unscale , 2" will scale the UI down by 1/2x or 0.5x.
e.g. "unscale , 1.5" will scale the UI back to normal if your computer is already scaling it up by 1.5x.
UI Scaling (edit)
Hey 
TabloidA, I added some features specifically to try to solve the screen scaling issue you're having with your monitor.
Use F5 to open the console, and type "showDisplayStats" (without quotes) and hit enter. (You may need to click back in the text field before hitting enter, it's finicky about focus sometimes.)
Anyway, you should get a popup showing some display stats. If you can tell me the value given for Device Pixel Ratio, that might help determine if DPR is related somehow.
After that, you can try the command "unscale , 1.5" (without quotes), to see if that shrinks the program from the mysterious 1.5x overscale back to the correct size.
If all else fails, for the time being the "scroll" command might help, as it allows you to scroll your view around the interface.