Issues Customizing the Ag-Grid Themes

In my case, it is used "ag-grid-enterprise": "^23.1.0" and "ag-grid-community": "^23.1.0" and for creating a custom theme for angular2+ application you should import to global style file(scss is in use for my case) several mixins file from ag-grid-community package and it will look like this:

@import '~ag-grid-community/src/styles/ag-grid';
@import '~ag-grid-community/src/styles/ag-theme-base/sass/ag-theme-base';

then you should include base theme with parameters set that you can override for this default theme:

.ag-theme-base {//should have specific name, since sizes doesn't work with custom name
  @include ag-theme-base(
    (
      foreground-color: black,
      background-color: yellow,
    )// this is parameters set object where you can ovveride ag-grid properties 
  );
}

List of the ag-grid parameters you can find link

Also, you can extract this parameter and apply for other elements(not sure that it is a useful option but ag-grid allows this)

.ag-header-cell {
  background-color: ag-param(foreground-color);
  color: ag-param(background-color);
}

Link to documentation about ag-param function link. Link to documentation regarding theme customization link.

Example of usage in angular:

<ag-grid-angular
  style="width: 100%; height: 400px;"
  class="ag-theme-base"
  ...
>
</ag-grid-angular>

Add ag-grid-overrides.scss file and put the saas variable you want to modify for the ag-grid theme. Full list of sass variables available can be found in this link - https://github.com/ag-grid/ag-grid/tree/master/src/styles. Import ag-grid-overrides.scss in your component.ts file. You can still have your own .css file for each component as shown below in app.component.css file.

app.component.ts

import '../ag-grid-overrides.scss';

app.component.html

<ag-grid-angular class="data-grid ag-theme-fresh" [gridOptions]="gridOptions">
</ag-grid-angular>

ag-grid-overrides.scss

// Customize the look and feel of the grid with Sass variables
// Up-to-date list of variables is available in the source code: https://github.com/ag-grid/ag-grid/blob/latest/src/styles/theme-fresh.scss 
$icons-path: "~ag-grid/src/styles/icons/";

// changes the border color
$border-color: #FF0000;

// Changes the font size
$font-size: 14px;

// Changes the font family
//$font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif;

// Reverts the cell horizontal padding change between ag-fresh and ag-theme-fresh
//$cell-horizontal-padding: 2px;

// changes the icon color
// $icon-color: red;
//$primary-color: green;

@import '~ag-grid/src/styles/ag-grid.scss';
@import '~ag-grid/src/styles/ag-theme-fresh.scss';

app.component.css (not required as this is a custom class on ag-grid-angular)

.data-grid {
  width: 500px; height: 300px; margin-bottom: 1rem;
}

angular-cli.json

"styles": [
        "../node_modules/ag-grid/dist/styles/ag-grid.css",
        "../node_modules/ag-grid/dist/styles/ag-theme-fresh.css",
        "styles.scss",
        "ag-grid-overrides.scss"
      ]